pee in my cornflakes 

someone goes and says or does something that ruins your day

"sorry i can't go this this weekend."
"why don't you just pee in my cornflakes won't cha"

"who peed in your cornflakes this morning?"
my mom won't let me go to the concert!"

pee in my cornflakes by brodybear May 9, 2007

Pee'd in my cornflakes 

Definition 1: A reply to be used when someone thinks they have the answer to a question but another individual participating in the argument proves them wrong with a detailed and knowledgeable answer with no sympathy for the other original participant's dignity or reputation.

Definition 2: A reply to be used when someone replies to an insult or argument with a flawlessly epic one liner or put down.

Example: "You've just pee'd in my cornflakes."

Pee'd in my cornflakes by Nathan Speirs... May 16, 2010