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]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "infallible":
   absolute, acceptable, agreeable, beyond all praise, certain,
   chaste, close, constant, correct, defectless, delicate, dependable,
   direct, effective, efficacious, efficient, even, exact, express,
   faithful, faultless, fine, flawless, foolproof, handy, helpful,
   ideal, immaculate, impeccable, indefectible, indefective,
   inerrable, inerrant, irreproachable, just right, mathematical,
   micrometrically precise, microscopic, nice, oracular, peerless,
   perfect, pinpoint, precise, pure, refined, reliable, religious,
   religiously exact, rigid, rigorous, satisfactory, satisfying,
   scientific, scientifically exact, secure, severe, sinless,
   spotless, square, stainless, strict, subtle, sure, surefire,
   taintless, unadulterated, unblemished, uncontaminated,
   undeceivable, undeviating, unerring, unfailing, unfaultable,
   unmistaken, unmixed, unspotted, untainted, useful