come hell or high water - any difficult situation or obstacle
ex. I will never leave you come hell or high water.
cock and bull story - an unbelievable story
ex. He said he was attacked by a vampire last night on his way home. That's a cock and bull story.
cast iron stomach - someone who can eat just anything without any ill effect
ex. Don't worry too much about what you would serve her for breakfast. Fragile as she may look, she has a cast iron stomach.