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1.
When you go out dancing and grind dance with someone from behind. The beating can be performed with your genitals and/or hands. A gentleman will always warm his hands up be rubbing them together before performing the spank.

Can also be abbreviated to "B-in B's"
Q: "What'd you do last night"
A: "We went to the club and were beating buns"

or

Q: "What were you doing out there?"
A: "Just B-ing B's"

or

Female: Damn I just want to go out tonight and get my buns beat.
by 3xMOP July 06, 2009