| covert(strange, unknown, curious, concealed, inexplicable) | |
| cryptic | |
| curious(adjective, strange, unknown, concealed, inexplicable) | |
| enigmatical | |
| equivocal(strange, unknown, curious, concealed, inexplicable) | |
| esoteric | |
| fixed | |
| furtive(strange, unknown, curious, concealed, inexplicable) | |
| hang tough(expression, tough, difficult, wild, awkward) | |
| hard | |
| hard-line(tough, difficult, wild, awkward, obstinate) | |
| hidden | |
| immovable(tough, difficult, obstinate, stubborn, willful) | |
| inaccessible | |
| inapprehensible | |
| incompliant(tough, difficult, wild, awkward, obstinate) | |
| incurable(tough, difficult, wild, awkward, obstinate) | |
| indecipherable(adjective, unable, answered) | |
| indeterminable | |
| indocile(tough, difficult, wild, awkward, obstinate) |
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| APA | Smart Define. (2015). insoluble thesaurus. Retrieved September 26, 2026, from https://smartdefine.org/insoluble/thesaurus/synonyms |
| Chicago | Smart Define. 2015. "insoluble thesaurus" https://smartdefine.org/insoluble/thesaurus/synonyms (accessed September 26, 2026). |
| Harvard | Smart Define 2015, insoluble thesaurus, Smart Define, viewed 26 September, 2026, <https://smartdefine.org/insoluble/thesaurus/synonyms>. |
| MLA | Smart Define. "insoluble thesaurus" 14 April 2015. Web. 26 September 2026. <https://smartdefine.org/insoluble/thesaurus/synonyms> |