Hidden in plain English
In current English a secret is a fact, plan, or formula known to one person or a small circle and withheld from everyone else. As an adjective the same word marks what is concealed, unofficial, secluded, or reserved for the initiated. The pair feels ordinary. Its history is a story of being set apart.
A late medieval loan
The origin of the word secret is a borrowing, not a native English growth. Old English already had ways to talk about the hidden: dēagol, and the related noun dēagolnes, covered what was concealed, obscure, even dark. Those forms later faded. What took their place arrived in the late fourteenth century, in Middle English.
Historical dictionaries agree on the Latin source and differ slightly on the last hop into English. American Heritage and Oxford treat Middle English secret as coming from Old French secret, itself from Latin. Other etymologies trace the English noun more directly to Latin secretum and the adjective through Old French. The disagreement is about the immediate channel, not about the older form.
That Latin noun named secrecy, a mystery, a hidden thing, a private conversation, even a stretch of solitude. It is the noun use of secretus: set apart, withdrawn, hidden, private. Old French already used the word as both noun and adjective, and English took both uses around the same time. An older French-shaped variant, secre, was common in English from the fourteenth century into the sixteenth and was at first the more frequent spelling. It did not survive.
The Middle English record shows how quickly the loan filled several slots: a fact kept confidential; a divine or natural mystery; a craft method known only to people already in the work; and, in church use, a prayer said in a low voice after the Offertory. A learned book title, the Secretum Secretorum, a treatise once attributed to Aristotle, kept the Latin shape in view for English readers.
Romance languages still carry close relatives: French secret, Spanish secreto, Italian segreto, Portuguese segredo, Catalan secret, Romanian secret. Those are sibling forms from the same Latin stock. German Geheimnis and Dutch geheim look like semantic matches, but they come from a different family of words for home and what belongs to the household. They are not cognates.
Set apart, then sieved
The Latin verb behind the adjective is secernere: to set apart, divide, exclude. The morphology is transparent. The prefix se- means apart, aside, on one’s own. It is related to the Latin reflexive pronoun and, further back, to a Proto-Indo-European pronoun root reconstructed as *s(w)e-, the same family that yields German sich. The second half is cernere, to separate, sift, or distinguish.
Cernere belongs to a large Indo-European group from a root usually written *krei- or *krey-. The oldest picture is a sieve: shake material so that one thing falls through and another stays. From sifting came judging and deciding. Greek krinein, to separate, decide, or judge, is a cognate, and English has borrowed a cluster of its descendants: crisis, critic, criterion, hypocrisy. From Latin cernere and its participle come certain, discern, discrete, concern, decree, and, less obviously, crime. Old English had a native member of the same root in hriddel, a sieve, later riddle in the sense of a coarse screen rather than a puzzle.
So the English word is not an opaque blob. It is se- plus the past participle of cernere: something sifted out and placed to one side. The physical image is separation, not whispering.
That split also explains a pair of look-alikes that older commentators liked to mash together. English sacred comes from Latin sacer, a different word with a different root, usually tied to making holy rather than to sieving. The cultural parallel—both the holy and the hidden can be “set apart from the common”—is real enough. The etymology is not. They are not the same formation.
A closer relative is secretary. Late Latin secretarius named a person attached to secretum: originally someone trusted with private business, later a clerk who kept records and correspondence. English had that official sense by about 1400. The writing desk called a secretaire, and the secretary bird named for a crest that looks like a pen tucked behind an ear, are later offshoots of the administrative line.
From mystery to method
The earliest English noun, in the late fourteenth century, leaned toward what is hidden from human understanding: a mystery in the theological or natural sense. By the early fifteenth century it also meant what is hidden from general knowledge. Mid-century usage adds the familiar social meaning: something studiously concealed, something that ought not to be revealed. The idiom of keeping a secret is recorded from that same mid-fifteenth-century window.
A technical sense ran beside the personal one. Late in the fifteenth century the word could name a method or process reserved for the initiated—alchemy, a craft, a science. From that idea, by 1738, comes the modern “key” or principle by which a difficulty is solved: the secret of a recipe, the secret of a long career. The word still names both the withheld fact and the knack that looks like a withheld fact.
Other senses thinned out. From the 1530s, secrets in the plural could mean parts of the body that propriety kept covered; that use is now archaic or comic. A rare verb, to secret something, meaning to hide it, appears in the 1590s and survives only as a curiosity beside the more usual secrete in that sense. Open secret, a fact everyone can learn if they bother, is a nineteenth-century phrase, recorded from 1828. Political compounds are later still: secret agent by 1715, secret service from 1737, secret police by 1823. In each of those, the old Latin idea of being set apart from public view is intact.
What the parts still do
If you want one date to hold onto, take the late fourteenth century: that is when English borrowed the Latin past participle, through French, and let it push aside native dēagol. The core has not wandered far from its parts. To keep something secret is still, etymologically, to set it apart—sifted out of the common pile and placed on its own.