Armor named for hide, not iron
A cuirass is the rigid covering that protects a fighter from the neck down to the waist: a breastplate joined to a matching plate for the back, or, in looser English, the breastplate alone. The same noun is also used for anything that works like that shell, from bony plating on some fish and reptiles to a medical jacket that encases the chest. The origin of the word cuirass is not a term from the forge. It is a word for leather.
From French kit lists into English
English borrowed the noun in the fifteenth century, while the usual spellings were still curas, curasse, curesse, and close variants. The immediate source was French: Old French cuirace or Middle French cuirasse. Dictionaries that date first English use place it in the middle of that century, with the written record clustering around 1425 to 1475. Middle English letters and inventories already treat the thing as ordinary equipment. A Paston letter from 1450 lists curesse among the arms of a thousand people arrayed for war. An armory inventory a few years later speaks of a pair of curasses, because the front and back plates were often counted as a matched set rather than as one fused object.
French did not invent the idea from scratch. Independent etymologies agree that the Romance noun goes back to Late Latin coriacea, originally the feminine of an adjective meaning made of leather, and used for a leather garment. Some reconstructions supply an implied vestis, a garment of hide. They disagree, modestly, about the last hop into French. Several standard dictionaries treat Old Occitan coirassa (Older labels say Old Provençal) as the likely model, with French then reshaping the form under the pull of cuir, the everyday French word for leather. Other accounts leave room for Italian corazza or Iberian relatives to have played a similar part. None of those routes changes the Latin core. They only describe how a shared adjective became a piece of military vocabulary as it moved through western Romance.
An older French name for comparable body armor, cuiriée (later quirie), came from a different Latin-type formation on the same hide-word and left a thin Middle English trail as quirie or quirre. That line did not become the modern English noun. Cuirass did.
Hide, a suffix, and a deeper cut
Latin corium means hide or leather. The adjective coriaceus (Late Latin also appears as coreaceus in some dictionary reconstructions) is that noun plus the suffix -aceus, the same ending English later borrowed as -aceous: of the nature of, made of. Once the implied noun, a garment, was obvious, feminine coriacea could stand alone. That is ordinary Latin morphology, not a later story invented to fit the armor. It is also why a metal breastplate still carries a leather name.
Corium itself belongs to a wide Indo-European family built on a reconstructed root (s)ker-, a verb of cutting and shearing. Native English shear is a cousin. Greek keirein, to cut or shear, sits nearby, as do Latin cortex (bark) and hide-words in other branches. The image is practical rather than poetic: leather is what you get when you cut the hide from an animal. French cuir is the same Latin hide, filtered through Gallo-Romance sound change, which is why French speakers could hear cuirasse as leather-gear even after smiths had replaced boiled hide with iron.
Romance cognates keep the silhouette of the Latin adjective: Italian corazza, Spanish coraza, Portuguese couraça, Catalan cuirassa. Germanic and Slavic languages later borrowed a French or neighboring form rather than inheriting a native hide-word for the same plate: German Kürass, Dutch kuras, Polish kirys, Russian kirasa. Those loans show the object traveling with French and Italian military fashion, not a shared Proto-Germanic armor term.
When the material changed and the name did not
Early cuirasses were leather, including hardened or boiled leather. By the time English was writing the word down, the typical field piece could already be iron or brass, two plates strapped or buckled together. The name did not follow the smith. It stayed with the silhouette: a close covering from throat to girdle.
English usage then split in two directions. Sometimes cuirass meant the combined pair; sometimes it meant the breastplate only, with the pair called cuirasses or a pair of cuirasses. In the sixteenth and seventeenth centuries a cluster of spellings such as curats and cuirats was often treated as a plural, which produced clipped singulars like curat and curate. Those truncated forms faded. Stress also moved. Early English put the weight on the first syllable; later, under French influence, it settled on the second, the pronunciation now usual in dictionaries.
The soldier named from the kit arrived later. Cuirassier, a mounted man in a cuirass, is recorded in English from the 1620s and became the proper label for a type of heavy cavalry that kept the breast-and-back plates long after infantry had shed most plate. Napoleonic troopers in shining iron still wore a word that, four centuries earlier, had meant a leather shirt.
Outside war, the metaphor is almost too obvious to miss. Zoologists call a carapace of bony plates a cuirass. Nautical French used the related noun for a ship's armor plating. Medicine borrowed the English word for a plaster jacket around the trunk and for a respirator that wraps the chest and pumps air by enclosing the ribcage. Each extension keeps the original picture: something that cages the torso.
What the spelling still admits
Strip the French polish off the modern English noun and you are left with Latin hide and a suffix that means made of. Museum plates with musket holes through both faces, and cavalry names that look purely martial, all sit on that older fact. English took the word in the fifteenth century, but the material named in it is older than the metal that made the object famous.