Noun
beast with two backs (plural beasts with two backs)
(idiomatic, euphemistic) Two people united in sexual intercourse in the missionary position.
I am one, sir, that comes to tell you your daughter and the Moor are now making the beast with two backs.(IAGO, ActI, SceneI) William Shakespeare
Hell, yes," Dev says, sitting up now. "Don't get me wrong - we're totally going to make the beast with two backs tonight. But if we do it right, it's going to feel like holding hands. David Levithan