From un- + mistake + -able.

unmistakable

1. Unique, such that it cannot be mistaken for something else.

   Antonyms: mistakable

   • 1895 May 7, H[erbert] G[eorge] Wells, chapter X, in The Time Machine: An Invention, New York, N.Y.: Henry Holt and Company, →OCLC ↗:

     Now, I still think that for this box of matches to have escaped the wear of time for immemorial years was a strange, and for me, a most fortunate thing. Yet oddly enough I found here a far more unlikely substance, and that was camphor. I found it in a sealed jar, that, by chance, I supposed had been really hermetically sealed. I fancied at first the stuff was paraffin wax, and smashed the jar accordingly. But the odor of camphor was unmistakable.

• French: reconnaissable entre tous, reconnaissable entre mille
• German: unverwechselbar, unverkennbar, unüberhörbar
• Italian: inconfondibile
• Portuguese: inconfundível
• Spanish: inconfundible