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When the person you're talking to is mumbling and you can't understand a damn thing they're saying. Usually used in situations where they're giving you important information like a reservation number or directions on how to get somewhere.
A typical mumblefuck conversation.
"Can I get your order please?"
"Yeah-um...I'd *mumble*...and then *mumble* large *unknown stammering*"
"I'm sorry, what?"

"Hey, Amy, how do we get to your house from downtown?"
"You'll go down *mumble mumble mumble*...after that you'll take a *mumble* onto 4th *mumble*
"Hold on, Amy, just a sec....Guys, can you shut up for a sec, this is a real mumblefuck conversation and all your talking isn't making this any easier to hear."
by Brett Burkhardt May 12, 2008
An unintelligible answer to a simple direct question.
My car salesman gave me a mumblefuck answer when I asked if the car was ever used as a taxi cab?

by BigjonnyC January 18, 2008
An extremely socially-awkward person, that rarely involves themselves in the coversation and only contributes to the conversation by randomly commenting with a semi-muffled mumble (this usually is followed by a very, very awkward silence within the whole group of people).
That kid Nile came over for the first time last night and was being a mumblefuck the whole time.
by corndonnus March 11, 2010
1. Trying to talk while engaging in oral sex.
Giver: Mfff mfff, whff dff ffffk?
Reciever: Quit yo mumblefuck, bitch.
by Kurt Jones May 01, 2005
Nickname for president George W. Bush
Mumblefuck spent over 7 minutes sitting on his ass on 9/11/01 after being informed of an attack on America.
by Mudesi September 12, 2006