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.
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>
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!"