Double backed beast 

A euphemism for coitus. This form is more common in popular culture than the original two backed beast. Predates Shakespeare.

Double backed beast... "These two did oftentimes do the two-backed beast together, joyfully rubbing and frotting their bacon 'gainst one another, in so far, that at last she became great with child of a fair son, and went with him unto the eleventh month; for so long, yea longer, may a woman carry her great belly, especially when it is some masterpiece of nature, and a person predestinated to the performance, in his due time, of great exploits."
François Rabelais (c. 1494 - 1553)
"Gargantua and Pantagruel"

Double backed beast by banquo_lives January 11, 2009