N. Old literary term for sexual copulation. The most famous being a quote from William Shakespeare's Othello where Iago informs the King that his daughter has been having sex with a black man. Though modern high school students generally assume that making the two back beast is a reference to doing it doggy style the literal meaning in the 15th century was sex face to face or in the missionary position.
"If you tell Momma I was spying on you, then I'll tell her you and Jorge were making the two backed beast."
William Shakespeare. Othello. (1605): Iago: ' I am one, sir, that comes to tell you, your daughter and the Moor are now making-the-beast-with-two-backs .'