66 entries under underground topic. ×

12
confidential(adjective, close, show, intimate, secret)
10
secret(adjective, close, sensitive, intimate, secret)
8
restricted(adjective, close, show, secret, hidden)
6
hush-hush(noun, adjective, intimate, secret, private)
5
clandestine(adjective, secret, personal, undisclosed)
4
arcane(adjective, obscure, intimate, secret, private)
4
esoteric(obscure, secret, unknown, clandestine)
4
top-secret(noun, adjective, secret, clandestine)
4
top secret(noun, expression, adjective, idiom, sensitive)
4
undisclosed(obscure, unknown, personal, clandestine)
3
close(obscure, intimate, private, unknown, clandestine)
3
covert(adjective, secret, restricted, clandestine)
3
inside(adjective, intimate, secret, private, personal)
3
mysterious(obscure, unknown, clandestine, undisclosed)
3
under wraps(expression, idiom, sensitive, secret, clandestine)
2
cryptic(obscure, unknown, clandestine, undisclosed)
2
furtive(adjective, obscure, secret, unknown, clandestine)
2
hidden(adjective, private, personal, clandestine)
2
occult(obscure, unknown, clandestine, undisclosed)
2
secretive(clandestine, undisclosed)
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APASmart Define. (2015). classified thesaurus. Retrieved September 26, 2026, from https://smartdefine.org/classified/thesaurus/synonyms
ChicagoSmart Define. 2015. "classified thesaurus" https://smartdefine.org/classified/thesaurus/synonyms (accessed September 26, 2026).
HarvardSmart Define 2015, classified thesaurus, Smart Define, viewed 26 September, 2026, <https://smartdefine.org/classified/thesaurus/synonyms>.
MLASmart Define. "classified thesaurus" 14 April 2015. Web. 26 September 2026. <https://smartdefine.org/classified/thesaurus/synonyms>