Beast with two backs

Making the beast with two backs is a euphemistic metaphor for two persons engaged in sexual intercourse. It refers to the situation in which a couple—in the missionary position, on their sides, kneeling, or standing—cling to each other as if a single creature, with their backs to the outside.

In English, the expression dates back to at least William Shakespeare's Othello (Act 1, Scene 1, ll. 126–127, c. 1601–1603):

I am one, sir, that comes to tell you your daughter and the Moor are now making the beast with two backs.

The earliest known occurrence of the phrase is in Rabelais's Gargantua and Pantagruel (c. 1532) as the phrase la bête à deux dos. Thomas Urquhart translated Gargantua and Pantagruel into English, which was published posthumously around 1693.

In the vigour of his age he married Gargamelle, daughter to the King of the 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.