GMs that insist on not having 'game terms' leak into roleplaying

[Ninjaxenomorph]

Hmm. I recall thoughts I had about what class names make sense a while back. I tend to draw some lines, but I wouldn't do this what your DM is doing. Spellcaster is an IC term in my games, since spells are clearly defined things in a world.

(I'm using Pathfinder classes because that's what I play)
Core
Barbarian- Could be used to describe any martial-type from the wilderness. Berserker could also be used.
Bard- Denotes a specific profession and a style of magic. Perfectly fine in-character, though minstrel works as well.
Cleric- Perfectly fine for any spellcasting clergyman. Fine in-character. Priest, preacher and the like also work.
Druid- What ELSE do you call a druid? It's an in-character designation as well as a class.
Fighter- Can be used to describe any martial-type, but this is one I'd personally shy away from, except maybe in certain circumstances ("He's a pretty good fighter, where did he train?"), and use the term 'warrior' instead. So more like current use of the word fighter in modern vernacular.
Monk- Could be used to both describe the secluded religious-types, as well as orders of martial artists. So, if someone was mechanically a PoW Stalker using Broken Blade and acted the part, he would be called a monk.
Paladin- I admit I'm pretty uptight about this in my games: somebody called a paladin in-character is, for all intents and purposes, a paladin class-wise (or else they are lying). While someone could be in a paladin order and not have paladin levels, I'd have them not actually have the title of paladin. The paladin could also be called a variety of generic words for martial fighter-types.
Ranger- I don't use them often, but when I do, it's typically a static job position in the wilderness surrounding a civilized settlement. You can call yourself a ranger, but NPCs will get the impression you were a ranger somewhere. The exception is a campaign where there was a military organization called the Rangers. Hunter could be used if they use a bow, though archer would be more common.
Rogue- Eh, this is a little more on the OOC side for me. Though it is fun watching players try to specify their expertise, that's half the fun of it. "I'm not a thief, I just have... roguish specialties."
Sorcerer- Any arcane caster could be called a 'mage' in my games, but sorcerers have a specific method of casting, drawing from their bloodlines. Depending on how learned someone is, they might refer to any mage as a sorcerer, like someone may call a physician a 'doc' instead of something specific, like a proctologist or something.
Wizard- Same as above: certain method of casting means it's an in-character designation open to interpretation. Any mage with a book of spells might be referred to as one, however.

Advanced Player's Guide
Alchemist- In-character term, though it can be used for for anyone that primarily uses alchemy, rather than just the magic-infused variation that the class uses. 'Chemist', 'madman', or 'crazy scientist' may also apply.
Cavalier- OOC term, unless the specific order is of cavaliers (in which case it's just any mounted warrior). Knights, riders, cavalry all work.
Inquisitor- IC term. Somewhat like the case of paladin above, but a little more loose. It's a title, most often filled by those of the inquisitor class, though the term may be different for certain religions. They may have a different position, as well, like preacher or priest.
Oracle- A bit hazy. The word could be used for any seer, but those in the know might refer to cursed divine casters as oracles. They could also be called a shaman or priest.
Summoner- Anyone that summons creatures using magic. Those bound to an eidolon are again sometimes called summoners by people with training, though 'mages' again rules the day.
Witch- Definitely an IC term, though any mage that stores spells in a familiar might be called one politely, or it could be a hostile term for anyone that uses magic. An unlearned commoner could refer to any weird magic as witchcraft, while a learned person might refer to anyone that gets their magic from a patron or a focusing on curses as a witch.

Base Classes
Gunslinger- IC term to describe any warrior that uses firearms (they tend to be a bit more common in my games), though it's a rare term. Definitely denotes some style, not just someone who carries a flintlock.
Magus- I prefer it as an OOC term. In-character, one might be referred to as a warrior mage or, more commonly, a spellsword. Generic warrior terms are also sometimes used.
Vigilante- Would be used as a term for anyone that takes the law into their own hands. The class could be called by anything, thought some of the archetype names might be used (a warlock that uses magic might be called a warlock in addition to being called a mage, etc, a serial killer would definitely be called a serial killer, though that is definitely not a unique term).

Advanced Class Guide (AKA where it gets tricky)
Arcanist- Unlikely to be called one, but it is a discipline of magic where someone with magic in their blood channels it towards study. Just calling them a mage is sufficient, thought the word 'arcanist' would be used to describe any arcane caster.
Bloodrager- Pretty much an OOC term, but I could see it being used as a term of self-identification. Preferably shouted. An obscure form of magic, but them being called berserkers is also likely.
Brawler- Just a word, OOC term. In a barfight, everyone's a brawler. Same terms as most other martials, though calling them a boxer is not out of the question.
Hunter- Like the magus, I don't like a hunter being called a hunter IC. Unless of course he's referring to it as a profession. I can't think of any names that really apply; they aren't druidic enough to be called druids, though I can see myself houseruling that to just make them called druids (they share the alignment restriction, but not the armor and the language).
Investigator- The class would be called anything an alchemist could be called if they are blatant about alchemy, and IC the term 'investigator' would be a job title. Investigators are rather good at doing it, though.
Shaman- Could be used IC, since their magic is unique (divine magic gained through spirits would be called shamanism), though the term could also be applied to any primitive divine caster.
Skald- An IC-term used by viking-types used for their storytellers. Very often used by actual skalds.
Slayer- OOC term. The word could of course be used.
Swashbuckler- A term more noting a type of lifestyle, I guess. I could see it being used to describe anyone that fits the bill, an extravagant warrior.
Warpriest- Could be used for any martial-oriented member of a clergy, though again, warpriests being called warpriests is pretty common.

Hybrid Classes
Antipaladin- Somewhat like the Paladin up above, though I see it having more 'internal' use. 'Overlord' comes forth as a fun word to use.
Ninja- IC term to describe anyone with ninja training from a ninja clan. I don't mind the ninjas, can't imagine why.
Samurai- A specific class of warrior that serves a lord in a Japanese-inspired setting; a fighter could be called one, and someone with the samurai class could be called something else. I've seen a Bushi Stalker also called one.

Occult Adventures (Phew!)
Kineticist- Yeah, I like the term OOC, but it's even a bit too modern for my tastes. "____mancers" would be the most common term applied to them.
Medium- Occupational term that can be applied to anyone that professionally channels spirits, but the Medium could be called that IC and I wouldn't raise a fuss, since they will almost always be channeling one.
Mesmerist- Like most of the other caster classes, it could be used as an IC designation. Can't really think of many good names to describe them otherwise, except perhaps 'psychic' and 'mentalist' for any psychic class.
Occultist- Pretty flatly an OOC term, though I can see it again being used as a way to designate what form of magic they practice.
Psychic- Called such by people referring to the casting method. Could just be called a mage or a mentalist.
Spiritualist- Used by people designating someone with a phantom, but more often they would actually be called a medium.

It gets really hazy with 3rd party classes, like some of Dreamscarred Press' output (I don't use psionics for the most part, so I'll just go over PoW and Akashic stuff)
Harbinger- OOC term. They might be called a 'fell warrior' IC, though that's more of something you might hear from a survivor of their attacks than a title.
Mystic- Sometimes used as an IC term to describe them, depending on how educated in magic the describer would be. 'Magical warrior' might be used instead, 'spellsword' to somebody that couldn't tell the difference between a supernatural maneuver and a spell.
Stalker- OOC term, though again it could be used to describe somebody that stalks. Assassin would be a frequent IC term.
Warder- OOC term, again. If somebody is called a warder, it would probably be used to describe someone putting up magical wards.
Warlord- OOC term, used frequently to describe people who IRL would be described as warlords, who of course could have Warlord levels.
Zealot- Never seen the class in play, and it's a fringe cases, since I don't really like Psionics, but I like the class. Definitely would not be called a zealot in description to what they do, but would be if they act like, well, a zealot.
Daevic- Like most other magic classes, this would be an IC designation to describe someone that has a bond with a Daeva.
Guru- Could describe someone with guru levels, depending on how they act and what they do. They could also be called monks.
Vizier- Definitely a designation, though many other generic mage terms would be used. The word 'vizier' would of course be used to denote the actual position.