| forbidding(adjective, strong, hard, tough, severe) |
| froward(adjective, persistent, obstinate, insistent, dogged) |
| frowning(adjective, strong, hard, tough, severe) |
| grim(adjective, strong, hard, tough, severe) |
| hang-tough(adjective, strong, hard, tough, severe) |
| hang tough(expression, tough, difficult, wild, awkward) |
| hard(adjective, strong, tough, severe, rough) |
| hard-core(adjective, strong, hard, tough, severe) |
| hard as nails(expression, adjective, idiom, tough, firm) |
| hardheaded(adjective, strong, hard, tough, severe) |
| harsh(adjective, strong, hard, tough, severe) |
| headstrong(adjective, hard, tough, severe, rough) |
| hell-bent(adjective, persistent, obstinate, insistent, dogged) |
| hidebound(adjective, persistent, obstinate, insistent, dogged) |
| immovable(adjective, strong, hard, tough, difficult) |
| implacable(adjective, strong, hard, tough, severe) |
| incompliant(tough, severe, difficult, wild, awkward) |
| incurable(tough, difficult, wild, awkward, obstinate) |
| indocile(tough, difficult, wild, awkward, obstinate) |
| insoluble(tough, difficult, wild, awkward, obstinate) |