Correct spelling, explanation: the root of the word unmistakably is a mistake. It comes from an Old Norse form mistaka, whose meaning used to be to take in error. When you add the -un prefix and the -ably suffix, you get the adverb unmistakably.

Definition of unmistakably:
1. adverb – when something is obvious and clear for somebody;
We’ve unmistakably taken the wrong turn and know we’re lost.

Other forms close to unmistakably:
Some other forms which come from the same word family as unmistakably are: mistake (noun/verb), unmistakable (adjective), mistakable (adjective), and mistaken (verb).

Incorrect spelling, explanation: a common mistake in the spelling of the word unmistakably is to write it as unmistakeably. The mistake occurs because many users of English tend to forget that the -e letter disappears from the word mistake when we add the -ably suffix. Therefore, the only correct version of the word is unmistakably.