Mean Ugly Mother Fucker. Usually referring to a gentleman of large physique, generally over 6ft with a Viking/biker like appearance, often astride a motorcycle.
When no riding like a twat can often found in biker bars, drinking copious amounts of alcohol and leading a life of debauchery and mayhem.
"Who's that big chap over there, hitting people?"
"Oh, it's a MUMF, best not to fuck with him"
Inflected forms: mumfed,mumfing
a. To screw something up beyond all hope of recovery or repair
b. An attempt to undo a clumsy action or poor decision that actually results in a more catastrophic result than if the original action or decision had been allowed to proceed.
Bob: "Did you see me spin out on turn 3"
Dave: "Yeah, you totally mumfed it"
As the coffee cup fell towards the ground he lunged forward to catch it and accidently kicked it through the glass window.