ironclad

ironclad

1. Fixed; inflexible; unbreakable. But not playing ball in the house is one of Mom's ironclad rules! I think any lawyer will have a hard time getting you out of that contract—it's ironclad.
2. Unable to be contradicted. I didn't even know what to say in response to the ironclad argument made by the opposing counsel.
Farlex Dictionary of Idioms.
See also:
  • fixed in (one's) ways
  • (one's) word is law
  • get on (one's) wick
  • get on somebody's wick
  • get on someone's wick
  • take a hard line
  • hard line
  • have a hard head
  • (one's) back is turned
  • do not hang noodles on my ears