A small protruding bulge of muscle between the thumb and pointer finger which develops on all great guitar players, most notably Jimi Hendrix, for which it was named. Non-guitar players do not exhibit any traces of this form of higher evolution, as its appearance can only be explained by countless hours devoted to the creation of rock and roll.
"What is that funny bump on your hand?"
"Oh, that is my hendrix muscle. It developed shortly after I nailed every metallica solo on the electric guitar."