IPA: /ˌɪn.kənˈtɛs.tə.bli/
KK: /ɪnˈkɒntɛstəblɪ/
In a way that cannot be disputed or questioned; undeniably.
The evidence presented in court proved the defendant's innocence incontestably.
Incontestably → It is formed from "in-" (meaning not), "contest" (from Latin "contestari", meaning to dispute or challenge), and "-ably" (meaning in a manner that can be). The word means in a manner that cannot be disputed or challenged.
Think of something that is 'not' ('in-') able to be 'disputed' ('contest') — that's why incontestably means something that cannot be challenged.