| rigorous(adjective, strong, hard, tough, severe) |
| rough(adjective, strong, hard, tough, harsh) |
| self-willed(tough, difficult, wild, awkward, obstinate) |
| serious(adjective, strong, hard, tough, severe) |
| set(adjective, persistent, obstinate, insistent, dogged) |
| single-minded(adjective, persistent, obstinate, insistent, dogged) |
| staunch(adjective, firm) |
| steely(adjective, strong, hard, tough, severe) |
| stiff(adjective, firm) |
| stiff-necked(adjective, strong, hard, tough, severe) |
| stouthearted(adjective, persistent, obstinate, insistent, dogged) |
| sturdy(adjective, strong, hard, tough, rugged) |
| tough nut(expression, obstinate, stubborn, willful) |
| unconquerable(adjective, persistent, obstinate, insistent, dogged) |
| undisciplined(tough, difficult, wild, awkward, obstinate) |
| unflinching(adjective, persistent, obstinate, insistent, dogged) |
| unpliable(tough, difficult, wild, awkward, obstinate) |
| unregenerate(adjective, persistent, obstinate, insistent, dogged) |
| unrelenting(adjective, strong, hard, tough, severe) |
| unsparing(adjective, strong, hard, tough, severe) |