to not give a damn; to not care one way or the other; to not give a shit; to not care what someone does given a choice of two options; the opposite of a chinese firecracker fart
originally from "The Soloist" when Robert Downey Jr's character tells Jamie Foxx's character nonchalantly, "I don't give a smooth fart whether your stay or go."
Dude 1: "Which computer are you gonna buy? Laptop or desktop?."
Dude 2: "Man, I don't give a smooth fart which one I get as long as they both play World of Warcraft and I can update my Facebook page."