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A butt-slapping is an act of slapping of a person's buttocks, usually out of jest.
Slapping your chin will cause an ebration and WILL PROBABLY HURT!
because your happy while ur slapping!! people just don't have other names for it. doh!!
It would depend on who you are slapping, and under the circumstances of the situation.
Chest-slapping by gorillas is an example of a threat display.