| impregnable(adjective, strong, secure, resolute, safe) |
| invincible(adjective, set, strong, certain, resolute) |
| incontestable(adjective, set, hard, certain, true) |
| indomitable(adjective, steadfast, unyielding, unconquerable) |
| inviolable(adjective, secure, safe, snug, intact) |
| secure(adjective, good, sound, sensible, safe) |
| strong(adjective, sound, secure, resolute, safe) |
| watertight |
| invulnerable(adjective, strong, sound, secure, resolute) |
| safe(adjective, clear, sound, secure, harm) |
| impenetrable(adjective, safety, untouchable, unconquerable, invincible) |
| incontrovertible(adjective, certain, true) |
| inexpugnable |
| inarguable(adjective, set, certain, true, definite) |
| irrefutable(adjective, certain, true) |
| unattackable(adjective, safety) |
| undeniable(adjective, certain, true, definite, proven) |
| unimpeachable(adjective, set, pure, certain, unquestionable) |
| unshakable |
| unyielding |