| stony(adjective, mean, hard, cool, severe) |
| callous(noun, adjective, mean, hard, cool) |
| stern(adjective, hard, easy, severe, rough) |
| stodgy(adjective, hard, difficult, complex) |
| viscous(adjective, feature, characteristic, structure, sticky) |
| adamant(adjective, mean, strong, hard, rough) |
| hardy(adjective, person, quality, body, strong) |
| indestructible(adjective, firm, rigid, unyielding, steely) |
| nasty(adjective, mean, bad, hard, cool) |
| obdurate(adjective, mean, strong, hard, cool) |
| ambitious(adjective, hard, difficult, complex) |
| involved(adjective, hard, difficult, complex) |
| massive(adjective, person, body, hard, difficult) |
| rock-hard(adjective, firm, rigid, unyielding, steely) |
| rowdy(noun, adjective, person, mean, attack) |
| byzantine(adjective, hard, difficult, complex) |
| disturbing(adjective, hard, severe, difficult, awful) |
| grave(adjective, hard, severe, difficult, harsh) |
| grinding(adjective, hard, difficult, complex) |
| hard to understand(expression, adjective, idiom, hard, difficult) |