unassailable

Impossible to dispute or disprove; undeniable.

Not subject to attack or seizure; impregnable.

Not assailable; incapable of being assailed; incontestable; hence, not to be moved or shaken from a purpose.

secure against attack; impregnable

undeniable, incontestable or incontrovertible

impossible to assail

without flaws or loopholes

immune to attack; incapable of being tampered with