unassailable

unassailable /ˌʌnəˈseɪləbəl/ adjective
unassailable
/ˌʌnəˈseɪləbəl/
adjective
Britannica Dictionary definition of UNASSAILABLE
formal
: not able to be doubted, attacked, or questioned

— unassailably

/ˌʌnəˈseɪləbli/ adverb