Freezing my a** off
Today I heard a farm girl discussing the birthing of calves in the snow. She said she'd never seen one die from the cold, but she had seen two "whose tails had frozen off." I wondered if that could be the source of the phrase "frezzing my a** off"?
Sounds good to me.
Then the girl saw two cows laughing their asses off and started that phrase too I suppose... then the oxen pulling the plow worked their asses off while the race horses were running their asses off in the paddock next door... ;-)