1. To disprove someone or make them sad. The term is usually used as part of friendly banter or by know it alls
Brian: Jet fuel can't melt steal beams right guys *laughs hysterically*
Dave: No need to piss in your cornflakes mate, but you're not funny
Dave: No need to piss in your cornflakes mate, but you're not funny
by Zorkkins August 2, 2016
Idiom meaning "Why are you in a bad mood?", implying discontent with the other person's attitude. As it is a rhetorical question, no response is expected. Often used in situations where the person being asked is angry as the result of an underlying cause and therefore snaps at people who have nothing to do with this cause.
Jordan: "How was your day, Hector?"
Hector: "Go to Hell, moron!"
Jordan: "Who pissed in your cornflakes?"
Hector: "Go to Hell, moron!"
Jordan: "Who pissed in your cornflakes?"
by suburbandefiner May 22, 2014