Open English Wordnet
Nouns
Adjectives
(s)ironclad, brassboundinflexibly entrenched and unchangeable“brassbound traditions”, “brassbound party loyalists”, “an ironclad rule”
(s)unassailable, unshakable, watertight, bulletproof, ironcladwithout flaws or loopholes“an ironclad contract”, “a watertight alibi”, “a bulletproof argument”