| in the dark(expression, idiom, concealment) | |
| irregular | |
| mysterious(adjective, obscure, secret, unknown, hidden) | |
| occult(noun, obscure, secret, unknown, hidden) | |
| privy | |
| radical(adjective, secret, subversive) | |
| sneaky(adjective, clandestine, undisclosed) | |
| stealthy(adjective, secret, hidden, spy, clandestine) | |
| sunken | |
| under an eclipse(expression, idiom, concealment) | |
| underfoot | |
| underhanded(secret, clandestine, undisclosed) | |
| wrapped in clouds(expression, idiom, concealment) | |
| agreement to disagree(expression, idiom) | |
| at odds with(expression, idiom) | |
| at variance with(expression, idiom) | |
| behind the curtain(expression, idiom) | |
| behind the scenes(expression, idiom) | |
| behind the veil(expression, idiom) | |
| furtively(adverb, undercover) |
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| APA | Smart Define. (2015). underground thesaurus. Retrieved September 22, 2026, from https://smartdefine.org/underground/thesaurus/synonyms |
| Chicago | Smart Define. 2015. "underground thesaurus" https://smartdefine.org/underground/thesaurus/synonyms (accessed September 22, 2026). |
| Harvard | Smart Define 2015, underground thesaurus, Smart Define, viewed 22 September, 2026, <https://smartdefine.org/underground/thesaurus/synonyms>. |
| MLA | Smart Define. "underground thesaurus" 3 November 2015. Web. 22 September 2026. <https://smartdefine.org/underground/thesaurus/synonyms> |