IPA: /ˌʌnəˈseɪləbli/
KK: /ʌnəˈseɪləbli/
In a way that cannot be attacked, questioned, or defeated.
The evidence presented was unassailably strong, leaving no room for doubt.
Unassailably → It is formed from "un-" (meaning not), "assail" (from Old French *assailir*, meaning to attack), and "-ably" (meaning in a manner). The word "unassailably" means in a manner that cannot be attacked or questioned.
Think of 'not' ('un-') being combined with 'attack' ('assail') to remember that 'unassailably' means something that cannot be attacked or questioned.