| implacable(adjective, mean, hard, tough, harsh) |
| incessant(adjective, persistent, constant, pitiless, unyielding) |
| indurate(adjective, mean, hard, tough, harsh) |
| intent(adjective, firm, determined, usual, regular) |
| inveterate(adjective, firm, determined, usual, regular) |
| iron(adjective, mean, hard, tough, harsh) |
| iron-fisted(adjective, persistent, constant, pitiless, unyielding) |
| ironclad(adjective, firm, determined, usual, regular) |
| locked in(expression, adjective, firm, determined, usual) |
| mean(adjective, hard, tough, harsh, inflexible) |
| mulish(adjective, mean, hard, determined, stubborn) |
| perpetual(adjective, persistent, constant, pitiless, unyielding) |
| perseverant |
| perseverative |
| persistent(adjective, determined, resolute, constant, pitiless) |
| perverse(adjective, mean, hard, tough, harsh) |
| pigheaded(adjective, mean, hard, determined, usual) |
| prearranged(adjective, firm, determined, usual, regular) |
| predetermined(adjective, firm, determined, usual, regular) |
| prescribed(adjective, firm, determined, usual, regular) |