From Latin, for "you, too."
WHY IT'S BAD
Suppose A is accused of terrorism. He reacts by accusing B, his enemy, of terrorism. Now, it's possible (but unlikely) that A actually chose this argument knowing he was totally innocent. More likely he wants to claim that his terrorism is PROVOKED. In effect, he's saying, "I have to do this, or I'm entitled to do this, because B did it first."
First, as logic it's a red herring. But what makes it douchebaggery rather than just another wartime propaganda tactic, is that it's MORALLY irrelevant as well as LOGICALLY irrelevant. The victims of terrorism almost never have any material control over either perpetrator ever.
ABU YAHYA: The reason I did is that I see all the time people using the rationale that, because somebody else did something bad to me, therefore I get to do something similar to anybody. It's sort of like sloppy revenge.
ANNA: Like men punishing random women because their girlfriends allegedly did something shitty to them?
ABU YAHYA: Actually, that's a perfect example of a tu quoque!