unassailable

unassailable (http://definr.com/unassailable)

     adj 1: able to withstand attack; "an impregnable fortress";
            "fortifications that made the frontier inviolable"
            [syn: impregnable, inviolable, secure, strong,
             unattackable]
     2: impossible to assail [syn: untouchable]
     3: without flaws or loopholes; "an ironclad contract"; "a
        watertight alibi" [syn: ironclad, unshakable, watertight]