I heard a wonderful one recently.
"if I can't help a 94 year old lady when she needs it then I'd best go and eat hay with the donkeys "
Thought it priceless.
my dad used to say something similar as an expression of disbelief or uselessness. i.e "if i can't get this fixed,i'll eat hay with a horse." thanks for reminding me-havent heard that phrase for about 30 years!!