The Collaborative International Dictionary of English v.0.48:

Indefeasible \In`de*fea`si*ble\, a. [Pref. in- not + defeasible:
   cf. OF. indefaisable.]
   Not to be defeated; not defeasible; incapable of being
   annulled or made void; as, an indefeasible or title.
   [1913 Webster]

         That the king had a divine and an indefeasible right to
         the regal power.                         --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indefeasible
    adj 1: not liable to being annulled or voided or undone; "an
           indefeasible right to freedom"; "an indefeasible claim to
           the title" [ant: defeasible]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "indefeasible":
   certain, changeless, constant, fated, fateful, immutable,
   inalienable, incommunicable, incommutable, inconvertible,
   ineluctable, inert, inescapable, inevasible, inevitable,
   inexorable, inflexible, insusceptible of change, intransmutable,
   invariable, irresistible, irretrievable, irreversible, irrevocable,
   lasting, necessary, noble, noncommunicable, noncontagious,
   noninfectious, nonreturnable, nonreversible, permanent, relentless,
   resistless, reverseless, sure, sure as death, sure as fate,
   unalterable, unalterative, unaltered, unavoidable, unchangeable,
   unchanged, unchanging, uncontrollable, undeflectable, undeviating,
   unimpartable, unmodifiable, unpreventable, unremitting,
   unrestorable, unreturnable, unstoppable, unsusceptible, unvariable,
   unvarying, unyielding



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INDEFEASIBLE. That which cannot be defeated or undone. This epithet is
usually applied to an estate or right which cannot be defeated.