look up any word, like donkey punch:
a laugh when something is not funny; a sound made when a joke or sarcasm is a flop; a laugh you can use after someone tells a joke they think is funny when its not; a response to a comment made by a douchebag, dick, or bitch.

can be pronounced differently in different situations; for example if something is really ridiculous you can hold out the mmmmms (mmmmmbuh) or put a bigger emphasis on the B. Should be said in a low voice.
person: "you must be a fan of mine"
me: "mbuh!"

person: tells a bad joke....etc
me: "mmmmBuh!"
by Kessler21 July 27, 2011