| unending(adjective, persistent, constant, continuous, endless) |
| unfaltering |
| unflagging |
| unmodifiable(adjective, set, secure, steady, fixed) |
| determined(adjective, hard, firm, resolute, rigid) |
| dogged(adjective, set, strong, hard, firm) |
| fast(adjective, calm, strong, solid, fixed) |
| intractable(adjective, set, hard, severe, rough) |
| ironclad(adjective, flexible) |
| long-lasting(adjective, stable, constant, enduring, lasting) |
| resolved |
| rigorous(adjective, hard, resolute, rigid, inflexible) |
| set(adjective, calm, strong, secure, firm) |
| stiff(adjective, set, hard, severe, rough) |
| timeless(adjective, lasting, permanent, everlasting, unending) |
| tough(adjective, calm, strong, rough, solid) |
| unbending(adjective, hard, firm, rigid, inflexible) |
| uncompromising(adjective, hard, secure, rough, firm) |
| undaunted |
| unflinching |