secretyoudaoicibaDictYouDict[secret 词源字典]
secret: [14] Etymologically, something that is secret is ‘separated’ from others, hence put out of the way, hidden. The word comes via Old French secret from Latin sēcrētus, an adjectival use of the past participle of sēcernere ‘separate’. This was a compound verb formed from the prefix sē- ‘apart’ and cernere ‘separate’ (source also of English certain, discern, excrement, etc).

From the 16th to the 18th centuries, secret was used as a verb, meaning ‘hide’, but it was then altered to secrete, on the model of Latin sēcrētus. (The other verb secrete, ‘produce fluids or other substances’ [18], is a back-formation from secretion [17], which goes back to Latin sēcrētiō ‘separation’, a derivative of sēcernere.) A secretary is etymologically a ‘secret’ or confidential helper.
=> certain, decree, discern, excrement, secretary[secret etymology, secret origin, 英语词源]
secret (v.)youdaoicibaDictYouDict
"to keep secret" (described in OED as "obsolete"), 1590s, from secret (n.). Related: Secreted; secreting.
secret (n.)youdaoicibaDictYouDict
late 14c., from Latin secretus "set apart, withdrawn; hidden, concealed, private," past participle of secernere "to set apart, part, divide; exclude," from se- "without, apart," properly "on one's own" (see se-)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" (see crisis).

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.