70 entries under underground topic. ×

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conspiratorial(clandestine, undisclosed)
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covered(clandestine, undisclosed)
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deep(clandestine, undisclosed)
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discreet(clandestine, undisclosed)
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disguised(clandestine, undisclosed)
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enigmatical(clandestine, undisclosed)
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incognito(clandestine, discreet, undisclosed)
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mystic(clandestine, undisclosed)
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mystical(clandestine, undisclosed)
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on the QT(expression, idiom, private, clandestine)
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out-of-the-way(clandestine, undisclosed)
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recondite(clandestine, undisclosed)
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reticent(clandestine, undisclosed)
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retired(clandestine, undisclosed)
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shrouded(clandestine, undisclosed)
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sly(clandestine, undisclosed)
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sneaky(clandestine, undisclosed)
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surreptitious(secret, clandestine, undisclosed, top-secret)
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unacknowledged(clandestine, undisclosed)
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under false pretense(expression, idiom, clandestine, undisclosed)
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APASmart Define. (2016). restricted thesaurus. Retrieved September 24, 2026, from https://smartdefine.org/restricted/thesaurus/synonyms
ChicagoSmart Define. 2016. "restricted thesaurus" https://smartdefine.org/restricted/thesaurus/synonyms (accessed September 24, 2026).
HarvardSmart Define 2016, restricted thesaurus, Smart Define, viewed 24 September, 2026, <https://smartdefine.org/restricted/thesaurus/synonyms>.
MLASmart Define. "restricted thesaurus" 26 February 2016. Web. 24 September 2026. <https://smartdefine.org/restricted/thesaurus/synonyms>