- Definitions|2
- Thesaurus|1.5k
- Abbreviations|0
| proof | |
| substantial(adjective, close, set, strong, hard) | |
| unassailable(adjective, safety) | |
| unclear(adjective, opaque) | |
| unyielding(adjective, set, hard, tough, firm) | |
| vague(adjective, opaque) | |
| ambiguous(adjective, doubtful, vague, unclear, hidden) | |
| cabalistic(adjective, obscure, solid, strange, odd) | |
| complex(adjective, difficult) | |
| concrete(adjective, set, hard, tough, solid) | |
| difficult(adjective, obscure, dark, strange, odd) | |
| dim(adjective, dark, gloomy, dense, cloudy) | |
| esoteric(adjective, obscure, difficult, strange, deep) | |
| hermetic(adjective, fast, short, dense, unaffected) | |
| indecipherable(adjective, obscure, deep, mysterious, incoherent) | |
| massive(adjective, density) | |
| mystifying | |
| profound | |
| thickset(thick, packed, density, compressed, crowded) | |
| uncomprehensible(adjective, knowledge, obscure, dark, superior) |
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| APA | Smart Define. (2015). impenetrable thesaurus. Retrieved September 23, 2026, from https://smartdefine.org/impenetrable/thesaurus/synonyms |
| Chicago | Smart Define. 2015. "impenetrable thesaurus" https://smartdefine.org/impenetrable/thesaurus/synonyms (accessed September 23, 2026). |
| Harvard | Smart Define 2015, impenetrable thesaurus, Smart Define, viewed 23 September, 2026, <https://smartdefine.org/impenetrable/thesaurus/synonyms>. |
| MLA | Smart Define. "impenetrable thesaurus" 10 April 2015. Web. 23 September 2026. <https://smartdefine.org/impenetrable/thesaurus/synonyms> |