A man and woman engaged in an act of sexual intercourse. An expression whose usage dates at least as early 1604, being mentioned in Shakespeare's Othello.
Your girlfriend and your bestfriend are making the beast with two backs.
by Lenee January 14, 2007
to have sex, having intercourse
wendy and marcus made the beast with two backs after serious foreplay
by Adriana G. April 21, 2006