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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
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