Open English Wordnet
Adjectives
(s)firm, steadfast, steady, stiff, unbendable, unfaltering, unshakable, unwaveringmarked by firm determination or resolution; not shakable“firm convictions”, “a firm mouth”, “steadfast resolve”, “a man of unbendable perseverence”, “unwavering loyalty”
(s)unassailable, unshakable, watertight, bulletproof, ironcladwithout flaws or loopholes“an ironclad contract”, “a watertight alibi”, “a bulletproof argument”