The Collaborative International Dictionary of English v.0.48:

Infallible \In*fal"li*ble\, a. [Pref. in- not + fallible: cf. F.
   infallible.]
   [1913 Webster]
   1. Not fallible; not capable of erring; entirely exempt from
      liability to mistake; unerring; inerrable. --Dryden.
      [1913 Webster]

   2. Not liable to fail, deceive, or disappoint; indubitable;
      sure; certain; as, infallible evidence; infallible
      success; an infallible remedy.
      [1913 Webster]

            To whom also he showed himself alive, after his
            passion, by many infallible proofs.   --Acts i. 3.
      [1913 Webster]

   3. (R. C. Ch.) Incapable of error in defining doctrines
      touching faith or morals. See Papal infallibility, under
      Infallibility.
      [1913 Webster]

WordNet (r) 3.0 (2006):

infallible
    adj 1: incapable of failure or error; "an infallible antidote";
           "an infallible memory"; "the Catholic Church considers
           the Pope infallible"; "no doctor is infallible" [ant:
           fallible]