IPA: /ˌʌn.mɪˈsteɪ.kə.bli/
KK: /ʌnˈmɪsteɪkəbli/
In a way that is clear and cannot be confused with anything else; certainly.
She was unmistakably the best player on the team.
Unmistakably → It is formed from "un-" (meaning not), "mistake" (from Old French *mistake*, meaning to take wrongly), and "-ably" (meaning in a manner that can be). The word means in a manner that cannot be taken wrongly or is clear and obvious.
Think of 'not' ('un-') taking something 'wrongly' ('mistake') — that's why unmistakably means something is clear and cannot be misunderstood.