PostOrgasmicStupidity. It's when you just had sex and whatever you say you end up feeling like a total idiot because you know none of it makes any sense and you just sound like a retard.
Girl 1: Omg last night I was having the WORST POS...I must have seemed so stupid.
Girl 2: I hate when that happens...
Verb: Derived from Piece Of Shit. Meaning someone dissed you or put you down or made you look bad. Essentially someone made you look like a piece of shit.
Joe POS'd you yesterday at Billy's house. He said you listen to Barry Manilow.