Well, Wiktionary gives me this:
Quote:
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From Middle English coppe, from Old English *coppe, as in ātorcoppe (“spider”, literally “venom head”), from Old English copp (“top, summit, head”), from Proto-Germanic *kuppaz (“vault, round vessel, head”), from Proto-Indo-European *gū- (“to bend, curve”). Cognate with Middle Dutch koppe, kobbe (“spider”). More at cobweb.
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So, cop, a spider. Atter apparently means "pus" or "poison".