IPA: /ʌnˈdɪbɪtəbli/
KK: /ʌnˈdɛbɪtəbli/
In a manner that cannot be argued against or disputed; clearly true or certain.
The results of the experiment undebatably support the theory.
Undebatably → It is formed from "un-" (meaning not) and "debatable" (from Latin "debatere", meaning to discuss or argue). The word means not open to discussion or argument, indicating certainty.
Think of 'not' ('un-') allowing for any 'discussion' ('debatable') — that's why undebatably means something is certain.