Etymology
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hidden (adj.)

past-participle adjective from hide (v.1); a Middle English formation (Old English had gehydd "hidden") on the model of ride/ridden, etc. As "secret, occult" from 1540s. Hidden persuaders (1957) was Vance Packard's term for ad men.

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cryptic (adj.)

1630s, "hidden, occult, mystical," from Late Latin crypticus, from Greek kryptikos "fit for concealing," from kryptos "hidden" (see crypt). Meaning "mysterious, enigmatic" is attested by 1920. Related: Cryptically.

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cryptonym (n.)

"private, secret, or hidden name," 1868 (by 1855 in German); from crypto- "secret, hidden" + -onym "name," from Greek (from PIE root *no-men-"name").

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covert (adj.)

"hidden, private, secret, concealed," c. 1300, from Old French covert (Modern French couvert) "hidden, obscure, underhanded," literally "covered," past participle of covrir "to cover" (see cover (v.)). Related: Covertly.

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latent (adj.)

mid-15c., "concealed, secret," from Latin latentem (nominative latens) "lying hid, concealed, secret, unknown," present participle of latere "lie hidden, lurk, be concealed," from PIE *late-, suffixed form of root *lādh- "to be hidden" (source also of Greek lēthē "forgetfulness, oblivion," lēthargos "forgetful," lathre "secretly, by stealth," lathrios "stealthy," lanthanein "to be hidden;" Old Church Slavonic lajati "to lie in wait for"). Meaning "dormant, undeveloped" is from 1680s, originally in medicine.

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latescent (adj.)

"tending to become latent or obscure, not obvious to perception," 1836, from Latin latescentem (nominative latescens), present participle of latescere "to hide oneself, be hidden," inchoative of latere "to lie hidden" (see latent). Related: Latescence.

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secret (n.)

late 14c., "that which is hidden from human understanding;" early 15c., "that which is hidden from general knowledge;" from Latin secretum "secrecy; a mystery; a thing hidden; secret conversation," also "retirement, solitude," noun from secretus "set apart, withdrawn; hidden, concealed, private." This is a past-participle adjective from secernere "to set apart, part, divide; exclude," from se- "without, apart," properly "on one's own" (see se-) + cernere "to separate" (from PIE root *krei- "to sieve," thus "discriminate, distinguish").

The meaning "something studiously hidden or concealed; what is not or should not be revealed" in English is from mid-15c. The sense of "key or principle by which some difficulty is solved" is from 1738, perhaps via the notion of "method or process hidden from the uninitiated" (late 15c.).

The alternative form secre, from an Old French variant form of secret, was common 14c.-16c. and seems to have been more frequent originally. It also was a variant of the adjective.

Open secret "matter or fact which is known to many; a secret which all who care to may learn" is from 1828. To keep (a) secret is from mid-15c. Secrets "parts of the body which propriety requires to be concealed" is by 1530s.

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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).

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Muscat 

capital of Oman, from Arabic Masqat, said to mean "hidden" (it is isolated from the interior by hills).

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