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, woman on top, on their sides, kneeling, or standing—cling to each other as if a single creature, with their backs to the outside.
|“||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. Othello was written c. 1601–1603.
The dictionary definition of beast with two backs at Wiktionary