unassailable

Secure against attack; impregnable.

Yet in the number, I do know but One That vnaſſayleable holds on his Ranke, Vnfhak’d of Motion : and that I am he, Let me a little ſhew it, euen in this : That I was conſtant Cymber ſhould be baniſh’d, And conſtant do remaine to keepe him ſo.

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.

Undeniable, incontestable or incontrovertible.

She won the debate with her unassailable logic.

So the French National Railways have secured a record which is likely, one imagines, to remain unassailable for a long time to come.

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