When it originated, this phrase was meant literally. An ironclad battleship was covered in iron and, when first built, was probably impervious to attack. Similarly, an ‘ironclad’ contract is unbreakable. An ‘ironclad’ promise can be believed wholeheartedly. Should you be arrested on ‘ironclad’ charges, the iron gates of prison will clang shut behind you.
But give this some thought: though bullets may not penetrate an ironclad ship, rust will. Iron may not be breakable, but a combination of air and water will disintegrate it, molecule by molecule. So what does this do to an ironclad contract?