| cliffhanger(difficulty, question, secret, puzzle, knot) |
| cold fish(expression, adjective, mean, hard, unfeeling) |
| crux(difficulty, question, secret, puzzle, knot) |
| difficulty(question, secret, puzzle, obscurity, subtlety) |
| dogged(adjective, mean, hard, harsh, inflexible) |
| firm(adjective, mean, hard, severe, harsh) |
| fixed(adjective, mean, hard, severe, harsh) |
| grabber(difficulty, question, secret, puzzle, knot) |
| hanging tough(expression, adjective, mean, hard, rigid) |
| hard(adjective, mean, severe, harsh, solid) |
| hard-boiled(adjective, mean, hard, tough, harsh) |
| hard-hearted(adjective, mean, hard, tough, harsh) |
| hard-nosed(adjective, mean, hard, tough, harsh) |
| harsh(adjective, mean, hard, severe, solid) |
| immovable(adjective, mean, hard, tough, harsh) |
| implacable(adjective, mean, hard, tough, harsh) |
| indurate(adjective, mean, hard, tough, harsh) |
| inexorable(adjective, mean, hard, harsh, stiff) |
| inflexible(adjective, mean, hard, severe, harsh) |
| inscrutability(difficulty, question, secret, puzzle, perplexity) |