The clapping of a woman's boobs when you're having sex with her while standing and her slightly bent over. As if they are applauding the performance. Size does matter. A good mirror can only help.
The term was created by pirates in the late 13th century, BC. Booty refers to a Pirate's "treasure" such as gold, chains, penor's etc. The clapping refers to an action that occurs after the "booty" was discovered. Pirates tend to clap in joy when a booty is found. Booty clap.