Source : Webster's Revised Unabridged Dictionary (1913)

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

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

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

   3. (R. C. Ch.) Incapable of error in defining doctrines
      touching faith or morals. See {Papal infallibility}, under
      {Infallibility}.

Source : WordNet®

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