that's Latin for...

You know when you're arguing with someone, and you know you're right and they're wrong—but all of a sudden you're defending yourself for something wrong that you did? And you're sitting in jail, thinking to yourself, "Wait. How did this happen? Tiffany's the one who said she could hot-wire the car in under 4 seconds. How come we're fighting about the time last year when I forgot to dope the doberman?"

It's a nasty bit of argumentative "tactics"—and it even has a name. "Tu quoque," Latin for "you, also."

Turns out there's a whole host of related argument derailers to be on the look-out, grouped under "ad hominem," which means (those Latin—busy, busy) "against the man."

There's the abusive ad hom way to go: "when an attack on the character or other irrelevant personal qualities of the opposition—such as appearance—is offered as evidence against her position." Often called the "red herring" argument.

Then you've got the circumstantial route: "in which some irrelevant personal circumstance surrounding the opponent is offered as evidence against the opponent's position.... The fallacy claims that the only reason why he argues as he does is because of personal circumstances, such as standing to gain from the argument's acceptance. "

If that's not enough, you can go with the ole reliable, Poisoning the Well:

To poison the well is to commit a pre-emptive ad hominem strike against an argumentative opponent. As with regular ad hominems, the well may be poisoned in either an abusive or circumstantial way.... Poisoning the Well is not, strictly speaking, a logical fallacy since it is not a type of argument. Rather, it is a logical boobytrap set by the poisoner to tempt the unwary audience into committing an ad hominem fallacy...

The underlying fallacy here is that the counter-arguments against the original topic are based on everything but actual arguments (past acts, circumstances, character)—when, as everyone knows, arguments and counter-arguments must stand on their own merit, regardless of the person advancing them (even if she did forget to dope the doberman).