A word with several lives
Today security names a condition: not being threatened in body, mind, money, or data. It also names the people and offices charged with keeping that condition, and, in finance, the stocks, bonds, and other claims that can be traded. A deposit can be security for a loan. None of these uses is a stretch if you know the older sense of something that makes a person or a bargain safe. They look like separate vocabulary only because later centuries split the jobs. The origin of the word security is less about locks than about a Latin idea of standing free of care.
Borrowed twice
The English noun is first recorded in the early fifteenth century as securite. It came from Old French securite and, at the same time, from Latin securitas, the abstract noun for being free from care. That French-and-Latin path is the form that stayed. It was not the first time the same Latin material had entered English. Around 1200, speakers already had sikernesse, and a little later sikerhede and sikerte, built on Middle English siker, which goes back to Old English sicor. Sicor itself was an earlier borrowing of Latin securus. The later, more obviously Latinate spelling crowded those native-looking words out of the standard language.
Surety is a doublet: the same Latin noun, reshaped while it travelled through French. French still keeps both sécurité and sûreté, a reminder that the family split more than once. The English adjective secure is later still, recorded from the 1530s, so the noun did not wait for a transparent English base plus the suffix -ity. Modern speakers parse security as secure with an ending, which is a fair description of how the word feels now, but historically the noun is the Latin abstract form, not a fresh derivation from the English adjective.
Francis Bacon and Elizabeth Barrett Browning used secureness; Thomas Browne used securement. Those rivals did not last. The -ity form won because it matched a large class of Latinate abstracts already in the language.
Without care
Latin securitas is the adjective securus plus -tas, the usual Latin way of naming a quality. The adjective splits cleanly: se- meaning apart from or without, and cura, care, trouble, attention. To be securus was to stand free of that burden. Latin could mean the word kindly, a quiet mind, or unkindly, a reckless ease. English inherited both tones.
The same idea appears in the Latin phrase sine cura, without care, which gave English sinecure, an office with pay and little duty. That is a parallel formation, not the same word, but it shows how ordinary the pieces were. Cura is also the source of English cure in the older sense of care for a patient or a parish, so the family is larger than it first looks. The prefix se- is the same element seen in separate and seclude: a movement away. Put together, the Latin word meant safety and also a mental state, quiet because nothing needed watching. Latin writers could use it for the safety of the commonwealth as well as for a person's ease. That double life, public and private, never quite left the word.
Romance descendants keep the same skeleton: French sécurité, Spanish seguridad, Italian sicurtà, Romanian securitate, Catalan seguretat, Portuguese seguridade. German Sicherheit looks like a cousin and is, in a roundabout way. German sicher comes from the same Latin adjective, borrowed into the Germanic languages, then given a native abstract suffix. Dutch zeker and related Scandinavian words for certainty sit in that same loan-group. They are not inherited Proto-Germanic siblings of the English noun; they are later neighbours who took the same Latin guest.
From ease to collateral to the state
The earliest English uses follow the Latin closely: a state of being safe from harm, and, by the mid-fifteenth century, freedom from anxiety. That second sense is now rare. It could even reverse. In early modern English the noun sometimes meant the opposite of watchfulness: a dangerous ease, the carelessness of people who think they are already safe. Shakespeare could use it that way in Richard II, where security is the remissness that lets a rival grow strong. The word that now names guards and locks once named the sleepiness that makes guards fail.
Legal English took the noun in another direction at about the same time. By the middle of the fifteenth century, security could be a pledge that an obligation would be met, first often a promise of good behaviour, then property or money put down against loss. From there it was a short step to a document that proved a debt or a right to payment, recorded by the 1680s, and to the modern financial sense of a tradable claim. When markets talk about securities in the plural, they are still talking about things that stand as backing, even if the backing is now a number on a screen.
The sense of something that makes a person or place safe is from the late sixteenth century. The specifically political sense, the safety of a state or a person as an object of policy, is much later, from about 1941. Capital-S Security as shorthand for officials or a ministry is later still, from the 1960s. Airport queues, badge-checked offices, and cybersecurity inherit that bureaucratic turn. A security check as a set phrase is attested by 1945. The figurative security blanket, the object you carry because it steadies you, is from 1966 and is tied to Linus in the Peanuts comic strip, whose blanket had been on the page since 1956.
What siker still tells us
If you want one fact that the later spelling hides, take siker. For centuries English already had a homegrown-looking word for the same idea, borrowed once from Latin and then rebuilt with native endings. Security is the second, more bookish arrival of the same root. The older forms died in standard English; the newer one picked up finance, police, and computing. The core image did not change: a life, a bargain, or a country standing without that extra care.