There are a few different ways to talk about retaliating against someone in equal terms. There's "an eye for an eye," "a tooth for a tooth," and "measure for measure," among others.
These phrases are all pretty transparent. If you take my eye, I'll take your eye. If you make that move, I'll make this move.
But what about "tit for tat?" One of English professor Anne Curzan's colleagues recently asked us about this one, and it's no wonder -- the meaning isn't nearly as obvious.