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] Unmiter

WordNet (r) 3.0 (2006):

unmistakable
    adj 1: clearly evident to the mind; "his opposition to slavery
           was unmistakable"
    2: clearly revealed to the mind or the senses or judgment; "the
       effects of the drought are apparent to anyone who sees the
       parched fields"; "evident hostility"; "manifest disapproval";
       "patent advantages"; "made his meaning plain"; "it is plain
       that he is no reactionary"; "in plain view" [syn: apparent,
       evident, manifest, patent, plain, unmistakable]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "unmistakable":
   absolute, admitting no exception, all-out, apodictic, apparent,
   bound, categorical, certain, clean-cut, clear, clear and distinct,
   clear as crystal, clear as day, clear-cut, coherent, complete,
   conclusive, connected, consistent, conspicuous, crisp,
   crystal-clear, crystalline, decided, decisive, defined, definite,
   definitive, determinate, direct, discernible, distinct, downright,
   entire, evident, explicit, express, final, fixed, flat, flat-out,
   glaring, global, implicit, in focus, inappealable, indisputable,
   indubitable, ineluctable, inevitable, limpid, loud and clear,
   lucid, luminous, manifest, necessary, noticeable, observable,
   obvious, open-and-shut, out-and-out, outright, palpable, patent,
   pellucid, perceivable, perceptible, peremptory, perfect,
   perfectly sure, perspicuous, plain, plain as day, positive,
   predestined, predetermined, prominent, pronounced, round, seeable,
   self-evident, self-explaining, self-explanatory, simple, staring,
   straight, straight-out, straightforward, sure, sure-enough,
   tangible, to be seen, total, translucent, transparent,
   transpicuous, true, unambiguous, uncircumscribed, unconditional,
   unconditioned, unconfused, undoubting, unequivocal, unhampered,
   unhesitating, univocal, unlimited, unmitigated, unperceivable,
   unqualified, unquestioning, unreserved, unrestricted, unwaivable,
   utter, visible, well-defined, well-marked, well-pronounced,
   well-resolved, whole, without exception, without reserve