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):^[1]

I am one, sir, that comes to tell you your daughter and the Moor are now making the beast with two backs.^[2]

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.^[3]

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.^[4]