Source : Webster's Revised Unabridged Dictionary (1913)

Incognizable \In*cog"ni*za*ble\, a.
   Not cognizable; incapable of being recognized, known, or
   distinguished. --H. Spenser.

         The Lettish race, not a primitive stock of the Slavi,
         but a distinct branch, now become incognizable.
                                                  --Tooke.

Source : WordNet®

incognizable
     adj : incapable of being perceived or known [syn: {incognoscible}]