He took the Bible verse about plucking out an eye or cutting off a hand if it is leading you into sin, so — being tempted by street prostitutes — he emasculated himself. This was when he was in his street preacher phase.
But we should not make fun of Boston Corbett. (It’s ok to make fun of Sean Penn.) As a boy back in England (or wherever he was from), he had been apprenticed to a hatter and grew up working with mercury. “Mad as a hatter” is a real thing.
Oops. He took the Bible verse literally. Being crazy.