| intractable(adjective, tough, firm, fixed, distant) |
| invariable(adjective, calm, strong, severe, stiff) |
| inveterate(adjective, firm, determined, usual, regular) |
| iron(adjective, mean, hard, tough, harsh) |
| ironclad(adjective, firm, cruel, determined, usual) |
| lasting(adjective, calm, strong, solid, steady) |
| long-lasting(adjective, stable, constant, enduring, lasting) |
| mean(adjective, hard, tough, harsh, unfeeling) |
| monolithic(adjective, set, firm, solid, hardened) |
| nailed(adjective, calm, strong, solid, steady) |
| never-failing(adjective, firm, solid, steady, substantial) |
| perdurable(adjective, calm, strong, solid, substantial) |
| perduring(adjective, stable, constant, vast, enduring) |
| perennial(adjective, stable, constant, enduring, lasting) |
| persistent(adjective, stable, constant, enduring, lasting) |
| pertinacious(adjective, determined, resolute, obstinate, uncompromising) |
| perverse(adjective, mean, hard, tough, difficult) |
| petrified(adjective, set, firm, solid, hardened) |
| pig-headed(adjective, determined, resolute, obstinate, uncompromising) |
| poised(adjective, calm, strong, secure, firm) |