1500-1600 "Beast with Two Backs" is a euphemism for sexual intercourse, either missionary style or standing. It is believed to be as old if not older in English from William Shakespeare's Othello.
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).
The origin of the euphemism is believed to be from 1532, appearing in Rabelais' Gargantua and Pantagruel originally in French.
Whereas the above example is believed to be written by Shakespeare himself in 1603.
I like the original version below best for its wit.
In the vigour of his age he married Gargamelle, daughter of the king of Parpaillons, a jolly pug, and well-mouthed wench. These two did oftentimes do the two-backed beast together, joyfully rubbing and frotting their bacon 'gainst one another.
Paraphrased from Wikipedia.