The Collaborative International Dictionary of English v.0.48:

Invulnerable \In*vul"ner*a*ble\, a. [L. invulnerabilis: cf. F.
   invuln['e]rable. See In- not, and Vulnerable.]
   [1913 Webster]
   1. Incapable of being wounded, or of receiving injury.
      [1913 Webster]

            Neither vainly hope
            To be invulnerable in those bright arms. --Milton.
      [1913 Webster]

   2. Unanswerable; irrefutable; that can not be refuted or
      convinced; as, an invulnerable argument.
      [1913 Webster]

WordNet (r) 3.0 (2006):

invulnerable
    adj 1: immune to attack; impregnable; "gunners raked the beach
           from invulnerable positions on the cliffs" [ant:
           vulnerable]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "invulnerable":
   armed, cloaked, copyrighted, covered, deathless, defended,
   dependable, fail-safe, firm, guaranteed, guarded, harmless,
   immortal, impenetrable, imperishable, impregnable, incontestable,
   incorruptible, indelible, indestructible, indomitable,
   ineffaceable, ineradicable, inexpugnable, inextinguishable,
   insuperable, invincible, inviolable, irresistible, nonperishable,
   overpowering, overwhelming, patented, policed, protected,
   quenchless, reliable, resistless, riskless, safe, safeguarded,
   screened, sheltered, shielded, sound, stable, steady, trustworthy,
   unassailable, unbeatable, unconquerable, undampable, undangerous,
   undefeatable, undestroyable, undying, unhazardous, unperilous,
   unprecarious, unquenchable, unrisky, unsubduable, unsurmountable,
   unyielding, warranteed