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
The Urban Dictionary Mug
One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.
Buy the mug