unassailable

Etymology

un- + assailable

adj

1. 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, Larry W. Riggs, “Moliere’s “Poststructuralism”: Demolition of Transcendentalist Discourse in Le Tartuffe”, in Symposium: A Quarterly Journal in Modern Literatures, volume 44, number 1, →DOI, page 53
   None of the processes, however, is either uniform or unassailable. The contradictions and distortions within any hegemonic discourse, as well as the discrepancies between it and the popular understandings of common sense, leave it ever vulnerable to penetration, criticism, and refusal. 1990, James Brow, “Notes on Community, Hegemony, and the Uses of the Past”, in Anthropological Quarterly, volume 63, number 1, George Washington U Institute for Ethnographic Research, →JSTOR, page 5
2. (by extension) Undeniable, incontestable or incontrovertible.
   She won the debate with her unassailable logic.

noun

1. Something, such as a belief, that cannot be assailed.