The Collaborative International Dictionary of English v.0.48:
Unmistakable \Un`mis*tak"a*ble\, a. Incapable of being mistaken or misunderstood; clear; plain; obvious; evident. -- Un`mis*tak"a*bly, adv. [1913 Webster] UnmiterWordNet (r) 3.0 (2006):
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, unmistakably, remarkably]