Sentence:  “Wa-al, old Jennifer, she don't care much for strangers anyhow, and she snorted like a steam engine and went straight up. That sombitch turned a double <WSD>fart-knocker</WSD> and lit flat on his back. He wasn't dead but he had the hound dog shit knocked out of 'im." 
Meaning of the word marked with <WSD> tags:  An instance of being thrown from, or hitting the ground after being thrown from, a horse.