quitter

"The good people of Missouri nominated me, and I'm not a quitter," Akin told Huckabee.

Mrs. Tyler issued a statement branding Clarke as a “weak-kneed quitter,” and repudiating him entirely.

But a girl who allows herself to take this attitude is a "quitter," and doesn't know the first principles of playing the game.

Quitter, kwit′ėr, n. a fistulous sore on the quarters or the heel of the coronet of a horse's hoof.