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®

unmistakable
     adj : clearly evident to the mind; "his opposition to slavery was
           unmistakable"