As the title says: What does やっちまえ mean?
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「やっちまえ」 is the tough guy's colloquial way of saying 「やってしまえ」 and it can mean many different things.
It can mean "Beat him up!", "Get him/her/them!", "Kill'em!", etc. It could even mean something I am not allowed to say on here.
Another possibility is when やる means "to give something to someone". In that case, 「やっちまえ」 means "Give it to (someone)!", which is much more peaceful than the first set of possible meanings above.
So, the best thing to do is to provide some context if none of the above seem to fit.