Clandestine

google

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mid 16th century: from French clandestin or Latin clandestinus, from clam ‘secretly’.

wiktionary

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From Latin clandestīnus(“secret, concealed”); compare French clandestin.

etymonline

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clandestine (adj.)

"secret, private, hidden, furtive," 1560s, from Latin clandestinus "secret, hidden," from clam "secretly," from adverbial derivative of base of celare "to hide" (from PIE root *kel- (1) "to cover, conceal, save"), perhaps on model of intestinus "internal." Related: Clandestinely. As a noun form, there is awkward clandestinity (clandestineness apparently being a dictionary word).