It's more along the lines of "Holier-than-thou" (isn't being holy a good thing?). Someone who makes a glorious crusade out of some percieved injustice that does not even directly affect them, usually when the people it actually affects don't care. The slur is the "warrior" part, not the "justice" part. I'll admit it's not the best word to describe it, but a true SJW is someone who goes on a multi-page rant on a forum about how someone is oppressing someone else by not using the right kind of language when no one else cares, least of all the person "Oppressed". Someone who makes a mountain out of a molehill while ignoring more urgent problems and who loves leaping to the defence of people who don't need it.
They aren't that rare on the internet. However, through general internet dickishness, it pretty much has come to mean "Anyone who cares about anything remotely social-related I don't care about".

*shrug*. It's the internet.