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Like the infamous "that's a dip" it means that a person who is being spoken to has made a mistake of something really obvious
after it is said the person who made the mistake is usually hit on the forehead
Matt is wearing a red shirt.
John: did you just see Matt's blue shirt?
Mark: his shirt is red. That's a Cooney
(forehead slap)
by mentalbaby32 June 11, 2008
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