| impenetrable(adjective, set, hard, tough, solid) |
| resistant(adjective, strong, tough, resist, sturdy) |
| impermeable(adjective, hard, solid, fast, short) |
| callous(noun, adjective, calm, bad, cold) |
| immune(adjective, resist, unaffected, unyielding, unbending) |
| inaccessible(adjective, unreasonable, useless, out, absurd) |
| impassable(adjective, solid, unreasonable, useless, absurd) |
| tight(adjective, proof, dense, unyielding, unbending) |
| hermetic(adjective, solid, fast, short, dense) |
| indifferent(adjective, calm, restrained, detached, apathetic) |
| insensible(adjective, bad, cold, indifferent, aloof) |
| insensitive(noun, adjective, bad, cold, detached) |
| invulnerable(adjective, secure, safe, unaffected, unyielding) |
| imperviable(adjective, unyielding, unbending, unable, unreceptive) |
| proof(adjective, calm, resist, restrained, fast) |
| unaffected(adjective, unresponsive, unyielding, unbending, unable) |
| hardened(noun, adjective, set, hard, tough) |
| unresponsive(adjective, calm, bad, cold, indifferent) |
| watertight(adjective, fast, short, unyielding, unbending) |
| airtight |