A loser, a person who cannot function whithout someone elses fist stuck up their ass. Also you see there lips moving but you don't believe the shit that comes out(as it comes from another person).
Lisa: Dad, what's a muppet?
Homer: Well, it's not quite a mop, and it's not quite a puppet, but MAN! Heh heh heh... So to answer your question, I don't know.
A Muppet is one of the best things in the world, it's really one of Jim Hensons creations. Some of the Muppets include:
Animal
SGT Floyd Pepper
Janice
Dr. Teeth
Zoot
Lips
Gonzo
Robin
The Swedish Chef Beaker
and many more