First attested in English by Shakespeare in Othello, 1603. Supposedly a translation of the French "la beste à deux doz" from Gargantua and Pantagruel, 1534, by François Rabelais.
beast with two backs (plural beasts with two backs)
- (idiomatic, euphemistic) Two people engaged in sexual intercourse.
- Typically used in the expression "make the beast with two backs".