[Petronski]

Ad hominem is an attack on the person having nothing to do with the argument. The charge of "liar" is directly relevant because it directly addresses the credibility of the speaker. It's called "impeaching credibility" and it is NOT ad hominem.

If I say Havoc is FAT, it is ad hominem because it is irrelevant, but if I say he is a liar (and if that statement is true), it most definitely is not.

[AndrewC]

The charge of "liar" is directly relevant because it directly addresses the credibility of the speaker.

Liar is a word. You must provide contrary evidence or proof against the argument. Calling someone a liar without the proof is pure ad hominem. And calling a person a liar with the proof is superfluous and therefore ad hominem(it is irrelevant)