It stands for Most Useless Person Possible Except Trainee
"That Steve is a total Muppet"
cookie monster: big bird, thats not weather
Big bird: okay then today's letter is acne
Cookie monster: big bird, that a word not a letter
Big Bird: okay today's letter is 3
Cookie Monster: big bird thats a number not a letter
Big bird: no, but its the number of joints i smoked today