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1.
Noun, term coined in 1984:

1. A person that replies to questions by mumbling.

2. A person's whose actions for a given situation are illogical or stupid to the point that one must comment about them.
Example 1:
Teacher: "So why did you think you could get away with downloading porn on the school's computers?"

Mumblyfuck: "Umm... well... *mumble*... that is... I ... *mumble*" <shakes heads>

Example 2:
Voter: "That politician just had a historical building torn down to make way for a museum to spotlight the city's history. What a mumblyfuck!"
by Dr-Mobius February 07, 2010