[adj] without flaws or loopholes; "an ironclad contract"; "a watertight alibi" [adj] impossible to assail [adj] able to withstand attack; "an impregnable fortress"; "fortifications that made the frontier inviolable"
impregnable, incontestable, incontestible, inviolable, invulnerable, ironclad, secure, strong, unattackable, unshakable, untouchable, watertight