unassailable /ˌʌnəˈseɪləbl̩/adjective 2noun 1forms 3 adjective2 •Secure against attack; impregnable.His recourse to written documents to make his discourse unassailable has created a virtually unalterable situation even as he is on the verge of being forced to “see through” Tartuffe. — 1990 •Undeniable, incontestable or incontrovertible.She won the debate with her unassailable logic. noun1 •Something, such as a belief, that cannot be assailed. formsunassailables · more unassailable · most unassailable