A slang term, used as a euphamism for sex. It is short for "Making The Two Backed Beast." It started as a paraphrase of Iago's line in Othello, Moor of Venice: "Your daughter and the Moor are now making the beast with two backs." It is most often used online, and not pronounce, rather all the letters said. However, some people do prefer to slur all the letters together in one word.
Can be used in the past, present or future tense.
Alternatively, it can be used as an insult.
We Mttbb last night.
They're Mttbb in the other room.
I'm hoping to Mttbb with him/her tonight.
Go Mttbb yourself.