Source : WordNet®

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"; "a bulletproof argument" [syn: {ironclad},
         {unshakable}, {watertight}, {bulletproof}]