The Collaborative International Dictionary of English v.0.48:

Impeccable \Im*pec"ca*ble\, a. [L. impeccabilis; pref. im- not +
   peccare to err, to sin: cf. F. impeccable.]
   Not liable to sin; exempt from the possibility of doing
   wrong. -- n. One who is impeccable; esp., one of a sect of
   Gnostic heretics who asserted their sinlessness.
   [1913 Webster]

         God is infallible, impeccable, and absolutely perfect.
                                                  --P. Skelton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impeccable
    adj 1: without fault or error; "faultless logic"; "speaks
           impeccable French"; "timing and technique were
           immaculate"; "an immaculate record" [syn: faultless,
           immaculate, impeccable]
    2: not capable of sin

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "impeccable":
   absolute, accurate, beyond all praise, blameless, chaste, clean,
   correct, defectless, destitute, errorless, exact, exquisite,
   faultless, flawless, ideal, immaculate, impecunious, impoverished,
   indefectible, indefective, indigent, infallible, irreproachable,
   just right, necessitous, needy, nice, peerless, penurious, perfect,
   poverty-stricken, precise, proper, pure, right, sinless, spotless,
   stainless, taintless, unadulterated, unblemished, uncontaminated,
   undefiled, unerring, unfaultable, unflawed, unimpeachable, unmixed,
   unprosperous, unsoiled, unspotted, unsullied, untainted, white