| secrecy(difficulty, question, secret, puzzle, obscurity) |
| secret(question, problem, puzzle, obscurity, knot) |
| set in stone(expression, adjective, idiom, mean, hard) |
| stiff-necked(adjective, mean, hard, tough, harsh) |
| subtlety(difficulty, question, secret, puzzle, obscurity) |
| thick-skinned(adjective, mean, hard, tough, harsh) |
| thriller(difficulty, question, secret, puzzle, perplexity) |
| tough(adjective, mean, hard, harsh, inflexible) |
| unbending(adjective, mean, hard, severe, harsh) |
| uncompassionate(adjective, mean, hard, tough, harsh) |
| uncompromising(adjective, mean, hard, severe, harsh) |
| uncooperative(adjective, mean, hard, tough, harsh) |
| unimpressible(adjective, mean, hard, tough, harsh) |
| unrelenting(adjective, mean, hard, severe, harsh) |
| unshakable(adjective, mean, hard, tough, harsh) |
| unsympathetic(adjective, mean, hard, tough, harsh) |
| unyielding(adjective, mean, hard, severe, harsh) |
| whodunit(difficulty, question, secret, puzzle, perplexity) |
| why(difficulty, question, secret, puzzle, knot) |
| bewitchment(perplexity, abstruseness, inscrutableness) |