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
by Zorkkins August 02, 2016
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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.
by Nathan Speirs... May 15, 2010
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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?"
by suburbandefiner May 21, 2014
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The phrase used for someone who can't drive well, meaning that their driving license couldn't have been official, because they're so bad at driving.
Look at that driver! He must have gotten his license from a cornflake packet!
by BelikovsRoza1697 December 26, 2010
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1. to be said when someone is acting really cheery

2. also can be used when someone is being very sappy
Example 1:

Person 1: THIS WORLD IS BEAUTIFUL AND EVERYONE IS JOYOUS!!!

Person 2: who sugar coated your cornflakes with unicorn piss this morning?

Example 2:

Person 1: god i love you so much i never want to leave you. you're my everything, without you i would be incomplete.

Person 2: who sugar coated your cornflakes with unicorn piss this morning?
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a hairy fanny that is dried up and crispy like a cornflake. and your fanny is un-usable.
nana, WOW, you have a leafy-cornflake
by penisdestroyer2006 May 03, 2021
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