Immune to attack; incapable of being tampered with.

synonyms: impregnable, inviolable, secure, strong, unattackable.

Roget 664: safe, secure, sure; in safety, in security; on the safe side; under the shield of, under the shade of, under the wing of, under the shadow of one's wing; under cover, under lock and key; out of danger, out of the woods, out of the meshes, out of harm's way; unharmed, unscathed; on sure ground, at anchor, high and dry, above water; unthreatened^†, unmolested; protected etc. v.; cavendo tutus [Lat.]; panoplied etc. (defended) 717^†.    snug, seaworthy; weatherproof, waterproof, fireproof.    defensible, tenable, proof against, invulnerable; unassailable, unattackable, impenetrable; impregnable, imperdible^†; inexpugnable; Achillean^†.    safe and sound etc. (preserved) 670; scathless etc. (perfect) 650; unhazarded^†; not dangerous etc. 665.    unthreatening, harmless; friendly (cooperative) 709.    protecting, protective etc. v.; guardian, tutelary; preservative etc. 670; trustworthy etc. 939.    ... show more

Impossible to assail.

synonym: untouchable.

Without flaws or loopholes.

synonyms: bulletproof, unshakable, watertight.