To break a horse is to train or domesticate it. This saying means a person is dangerous and mean--in essence, that he can't be "broken." Use it to convey that my-family-has- ranching-roots allure.

Meaner than a skilletful of rattlesnakes.

So low he'd steal the nickels off a dead man's eyes.

She makes a hornet look cuddly.

Meaner than a junkyard dog.

He'd start a fight at the drop of a hat--and he'd drop it himself.