Source : Webster's Revised Unabridged Dictionary (1913)

Unmistakable \Un`mis*tak"a*ble\, a.
   Incapable of being mistaken or misunderstood; clear; plain;
   obvious; evident. -- {Un`mis*tak"a*bly}, adv.

Source : WordNet®

unmistakably
     adv 1: without possibility of mistake; "this watercolor is
            unmistakably a synthesis of nature"
     2: in a signal manner; "signally inappropriate methods" [syn: {signally},
         {remarkably}]