Source : Webster's Revised Unabridged Dictionary (1913)

Inerrable \In*er"ra*ble\, a. [L. inerrabilis. See {In-} not, and
   {Err}.]
   Incapable of erring; infallible; unerring. ``Inerabble and
   requisite conditions.'' --Sir T. Browne. ``Not an inerrable
   text.'' --Gladstone.

Source : WordNet®

inerrable
     adj : not liable to error; "the Church was...theoretically
           inerrant and omnicompetent"-G.G.Coulton; "lack an
           inerrant literary sense"; "an unerring marksman" [syn:
           {inerrant}, {unerring}]