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1.
When your a with a woman in a confined space, usually after a couple drinks and she rejects your offer of sex. In a last ditch attempt, you take your index finger and proceed to stick it in her ass, lightly yelling "Ba-Dump!" in the process. Results may vary.
Kim: you know we're not going any futher right?

Joe: Are you sure.....Ba-dump!

Kim: get out
by swaggerchristopher October 28, 2011
 
2.
Your ass or your backside.
Wind your waist and shake your BaDump
by Boopy January 06, 2008