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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. © 2015 Farlex, Inc, all rights reserved.