Every so often, an obscure, rarely-heard expression appears in a prominent blog, and then suddenly, it's everywhere.
This week's example, tu quoque. I'm not sure whether I saw it first over on Althouse in comments, or somewhere else, but it's certainly making the rounds -- I even heard it on Rush this morning.
It's a useful expression, but not one I remember hearing before this past week. In online discussions, you'll often see tu quoque arguments lobbed when one party is pressing for a reasoned answer, and the other party has none: Well, you don't have any right to accuse me of X, because you've done Y!
Tu quoque arguments are more subtle than ad hominem attacks, because usually the target feels compelled to defend himself either against the charge that he's done Y, or whether Y is even anything to be ashamed of. The best thing to do if you find yourself in a tu quoque confrontration is to call attention to it: quit trying to change the subject!