Roomate: Oh crap dude! We need some kind of signal like a sock on the doorknob or something. I cant believe I walked in on you and your girl making the double backed monster.
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"
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.