Short for Pain In The Ass Cunt, a self-explanatory term for someone who really, really blows and should be avoided at all costs.
Useful because it sounds vaguely like a non-English word so can be misinterpreted as a compliment.
Thomas: Wow, did you see tha 'tude I was getting from that guy in the 7-11?
Frank: Totally. That loser is a total pitac.