That cannot be taken by assault or storm; incapable of being overcome, subdued, or overthrown by… figurative of a statement, argument, principle…

(un-, prefix1 affix 1b.)

Characteristic of or resembling Achilles; spec. invulnerable, invincible.

figurative. That cannot be overcome or vanquished; invincible, unconquerable, proof against attack.

Not assailable; not open to assault or attack. Also figurative.

figurative. Incapable of being damaged or injuriously affected by attack; not effectively assailable.

Not attackable; that cannot be attacked; unassailable.

That cannot be punctured.

(a) impervious to storm; also, protected from or affording protection from stormy weather; (b) proof against storming or assault; also figurative.