| plodding(adjective, strong, firm, determined, steady) |
| plugging(adjective, strong, firm, determined, steady) |
| resistless |
| restive(determined, irritable, fixed, restless, stubborn) |
| serious(adjective, strong, hard, tough, resolute) |
| severe(adjective, strong, hard, tough, persistent) |
| stable |
| steely(adjective, set, strong, hard, tough) |
| stoical(adjective, calm, cool, tolerant, enduring) |
| stouthearted(strong, bold, resolute, persistent, spirited) |
| thorough(adjective, resolute) |
| true(adjective, resolute, tenacious) |
| unbending(adjective, strong, hard, tough, difficult) |
| uncooperative(difficult, wild, awkward, persistent, obstinate) |
| undisciplined(difficult, violent, wild, awkward, stubborn) |
| undiscouraged(adjective, bold, unconcerned, brave, steadfast) |
| unfailing(adjective, strong, firm, determined, steady) |
| unrelenting(adjective, set, strong, hard, tough) |
| unremitting(adjective, strong, firm, determined, steady) |
| unshakeable(adjective, resolute) |