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2.
to copulate or partake in coital activity; to do the bone dance; have sex.
The saloon keeper and the town trollop were 'making the beast with 2 backs'like 2 jackrabbits on steroids atop the pool table, after he locked the saloon doors, as the last sot vacated the premises at 3 a.m.
by weave January 08, 2004
 
1.
Sexual intercourse.

The actual expression is hundreds of years old, and was made famous by Shakespeare in Othello.
BRABANTIO: What profane wretch art thou?
IAGO: I am one, sir, that comes to tell you, your daughter and the Moor are now making the beast with two backs.

(Othello, Act 1, Scene 1).
by Dr Pinch April 12, 2005