unassailable

from WordNet (r) 3.0 (2006)

unassailable
    adj 1: immune to attack; incapable of being tampered with; "an
           impregnable fortress"; "fortifications that made the
           frontier inviolable"; "a secure telephone connection"
           [syn: {impregnable}, {inviolable}, {secure}, {strong},
           {unassailable}, {unattackable}]
    2: impossible to assail [syn: {unassailable}, {untouchable}]
    3: without flaws or loopholes; "an ironclad contract"; "a
       watertight alibi"; "a bulletproof argument" [syn:
       {unassailable}, {unshakable}, {watertight}, {bulletproof}]

from The Collaborative International Dictionary of English v.0.48

Unassailable \Unassailable\
   See {assailable}.

from Moby Thesaurus II by Grady Ward, 1.0

18 Moby Thesaurus words for "unassailable":
      impenetrable, impregnable, incontestable, indomitable,
      inexpugnable, insuperable, invincible, inviolable, invulnerable,
      irresistible, overpowering, overwhelming, resistless, unbeatable,
      unconquerable, unsubduable, unsurmountable, unyielding