secret (n.)

late 14c., from Latin secretus "set apart, withdrawn; hidden, concealed, private," past participle of secernere "to set apart, part, divide; exclude," from se- "without, apart," properly "on one's own" (see se-) + cernere "separate" (from PIE root *krei- "to sieve," thus "discriminate, distinguish").

As an adjective from late 14c., from French secret, adjective use of noun. Open secret is from 1828. Secret agent first recorded 1715; secret service is from 1737; secret weapon is from 1936.

secret (v.)

"to keep secret" (described in OED as "obsolete"), 1590s, from secret (n.). Related: Secreted; secreting.

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Definitions of secret
secret (adj.)
conducted with or marked by hidden aims or methods;
a secret agent
secret missions
secret sales of arms
Synonyms: clandestine / cloak-and-dagger / hole-and-corner / hugger-mugger / hush-hush / surreptitious / undercover / underground
secret (adj.)
not open or public; kept private or not revealed;
secret talks
secret ingredients
a secret formula
secret (adj.)
not openly made known;
a secret marriage
a secret bride
Synonyms: unavowed
secret (adj.)
communicated covertly;
secret messages
their secret signal was a wink
secret (adj.)
not expressed;
secret (or private) thoughts
Synonyms: private
secret (adj.)
designed to elude detection;
the secret compartment in the desk
a secret passage
Synonyms: hidden
secret (adj.)
hidden from general view or use;
a secret garden
Synonyms: privy / secluded
secret (adj.)
(of information) given in confidence or in secret;
their secret communications
Synonyms: confidential
secret (adj.)
indulging only covertly;
a secret alcoholic
secret (adj.)
having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding;
the secret learning of the ancients
secret (adj.)
the next to highest level of official classification for documents;
secret (n.)
something that should remain hidden from others (especially information that is not to be passed on);
the combination to the safe was a secret
he tried to keep his drinking a secret
secret (n.)
information known only to a special group;
the secret of Cajun cooking
Synonyms: arcanum
secret (n.)
something that baffles understanding and cannot be explained;
it remains one of nature's secrets
Synonyms: mystery / enigma / closed book