Corrugator Supercilii
(kor'ah-ga-ter soo"per-si'le-i)
(corrugo=wrinkle; supercilium=eyebrow)

Sure, when you break down the word it means what it does, but when you say it I can't help but laugh. The next time I furrow my eyebrows I'll probably think of the muscles I'm using to cheer me up. LOL! I'm a dork!