Etymology
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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late 14c., from Latin secretus "set apart, withdrawn; hidden, concealed, private," past participle of secernere "to set apart, part, divide; exclude," from se- "without, apart," properly "on one's own" (see se-)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.
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secret (v.)
"to keep secret" (described in OED as "obsolete"), 1590s, from secret (n.). Related: Secreted; secreting.
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secretly (adv.)
early 15c., from secret (adj.) (see secret (n.)) + -ly (2).
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secretive (adj.)
"inclined to secrecy," 1815 (implied in secretiveness); see secret (n.) + -ive. The word also was in Middle English with a sense "secret, hidden" (mid-15c.). Related: Secretively.
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secrecy (n.)
1570s, from secretee, "quality of being secret" (early 15c.), from Old French secré, variant of secret (see secret (n.)) + -ty (2). Form altered on model of primacy, etc.
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secretion (n.)
1640s, "act of secreting;" 1732, "that which is secreted," from French sécrétion, from Latin secretionem (nominative secretio) "a dividing, separation," noun of action from past participle stem of secernere "to separate, set apart" (see secret (n.)).
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seclude (v.)
mid-15c., "to shut up, enclose, confine," from Latin secludere "shut off, confine," from se- "apart" (see secret (n.)) + -cludere, variant of claudere "to shut" (see close (v.)). Meaning "to remove or guard from public view" is recorded from 1620s. Related: Secluded; secluding.
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separate (v.)

late 14c., from Latin separatus, past participle of separare "to pull apart," from se- "apart" (see secret (n.)) + parare "make ready, prepare" (from PIE root *pere- (1) "to produce, procure"). Sever (q.v.) is a doublet, via French. Related: Separated; separating.

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sedulous (adj.)
1530s, from Latin sedulus "attentive, painstaking, diligent, busy, zealous," probably from sedulo (adv.) "sincerely, diligently," from sedolo "without deception or guile," from se- "without, apart" (see secret (n.)) + dolo, ablative of dolus "deception, guile," cognate with Greek dolos "ruse, snare." Related: Sedulously; sedulousness.
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select (adj.)
1560s, from Latin selectus, past participle of seligere "choose out, single out, select; separate, cull," from se- "apart" (see secret (n.)) + legere "to gather, select," from PIE root *leg- (1) "to collect, gather." The noun meaning "a selected person or thing, that which is choice" is recorded from c. 1600. New England selectman first recorded 1640s.
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