unshakable

adjectives

marked by firm determination or resolution; not shakable
"firm convictions"; "a firm mouth"; "steadfast resolve"; "a man of unbendable perseverence"; "unwavering loyalty"
firm, steadfast, steady, stiff, unbendable, unfaltering, unshakable, unwavering
without flaws or loopholes
"an ironclad contract"; "a watertight alibi"; "a bulletproof argument"
bulletproof, unassailable, unshakable, watertight