| diehard |
| difficult(tough, wild, awkward, obstinate, stubborn) |
| distant(adjective, tough, firm, fixed, rigid) |
| do or die(expression, adjective, idiom, tough, firm) |
| dour |
| dug in(expression, adjective, tough, firm, distant) |
| formal(adjective, tough, firm, fixed, distant) |
| froward |
| frozen(adjective, hard, firm, solid, tense) |
| graceless(adjective, hard, firm, solid, tense) |
| hang tough(expression, tough, difficult, wild, awkward) |
| hard-line(tough, severe, difficult, wild, awkward) |
| hard as nails(expression, adjective, idiom, tough, firm) |
| hardened(adjective, hard, firm, solid, stiff) |
| hardheaded(adjective, stubborn) |
| hold the fort(expression, adjective, idiom, tough, firm) |
| hold the line(expression, adjective, idiom, tough, firm) |
| immalleable(adjective, hard, firm, solid, stiff) |
| impliable(adjective, hard, firm, solid, stiff) |
| incurable(tough, difficult, wild, awkward, obstinate) |