| regular(adjective, firm, determined, usual, planned) |
| remorseless(adjective, persistent, constant, pitiless, unyielding) |
| resistant(adjective, determined, resolute, uncompromising, unshakeable) |
| resolved(adjective, firm, determined, usual, regular) |
| rooted(adjective, firm, determined, usual, regular) |
| scheduled(adjective, firm, determined, usual, regular) |
| settled(adjective, firm, determined, usual, regular) |
| solid as a rock(expression, adjective, idiom, firm, determined) |
| specified(adjective, firm, determined, usual, regular) |
| stated(adjective, firm, determined, usual, regular) |
| staunch |
| steady(adjective, determined, resolute, persistent, constant) |
| stipulated(adjective, firm, determined, usual, regular) |
| tenacious(adjective, determined, vigorous, resolute, persistent) |
| thick-skinned(adjective, mean, hard, tough, harsh) |
| tough(adjective, mean, hard, harsh, careful) |
| tough nut |
| tough nut to crack(expression, adjective, idiom, mean, hard) |
| unabated(adjective, persistent, constant, pitiless, unyielding) |
| unbroken(adjective, persistent, constant, pitiless, unyielding) |