Two spellings that are really two words
In present-day English, case can name a situation, a lawsuit, a medical instance, a grammatical form, and also a box or covering. Those jobs sit together in one dictionary lemma, but they did not grow from a single ancestor. English folded two separate borrowings into the same spelling and the same pronunciation. Later senses then branched from each line on its own.
The first family covers what happens: a particular instance, the facts of a matter, a court proceeding, a patient, or the inflected shape of a noun. The second family covers what holds something: a suitcase, a phone shell, a window frame, a tray of type. Treating them as one busy word is convenient for speakers. Historically they come from different Latin verbs and different Indo-European roots.
A falling that became an event
The situation sense reached Middle English from Old French cas, a noun for an event, a happening, a quarrel, or a trial. French had taken it from Latin cāsus, which meant a chance, an accident, or an occasion, and, more literally, a falling. That noun is built on the past-participial stem of cadere, to fall, sink, settle, or decline.
Speakers already linked falling with happening. English still keeps that picture in befall. What drops upon a person is what occurs. By the early thirteenth century the borrowed noun could mean a state of affairs or whatever happens to someone. Around 1300 it is used for an instance or example. By about 1400 it can mean the actual condition of things. Late in the fourteenth century it is established in law as a piece of litigation and in medicine as one occurrence of a disease. The same period gives the grammatical sense: one of the forms that mark a noun’s role in a clause.
References do not line up on a single first date. Etymological surveys often treat the everyday French loan as a thirteenth-century arrival. Other dictionaries note an earlier learned use: Old English already had cāsus for grammatical case, taken straight from Latin. The Oxford English Dictionary describes the noun as partly from Latin and partly from French, with the oldest English use in the Old English period. The safest picture is a learned Latin term for grammar, then a wider French borrowing that filled out the language of events, arguments, and examples.
Why grammar talks about falling
Latin used cāsus for noun inflection because Greek had already named those forms ptōsis, itself a falling. The image is of a word standing upright in the nominative, then dropping or bending into other roles. English inherited that metaphor through Latin, not through a native Germanic label for the same idea. Related English words from the same Latin verb include chance, casual, accident, cadence, cascade, decay, incident, and occasion. Further back, the reconstructed Proto-Indo-European root is often given as *kad-, to fall. Sanskrit has a related verb for falling down. That cluster is independent of the box-word discussed below.
A box that holds what you take
The container sense is younger in ordinary English by about a century, appearing early in the fourteenth century. It comes through Anglo-French and Old Northern French casse. Central Old French had chasse, a case or reliquary; Modern French keeps châsse in that line and also caisse for a chest or till. The Latin source is capsa, a box or repository, especially one for books or scrolls, from capere, to take or hold.
That is a different verb and a different Indo-European root, usually reconstructed as *kap-, to grasp. Romance cousins make the family obvious: Spanish caja, Portuguese caixa, Italian cassa. English cash and the hunting word chase sit in this neighborhood as doublets, not as children of cadere. By the late fourteenth century the noun can mean an outer protective covering. Later it names a frame, as in casement, and a shallow tray of metal type.
Latin capsa is sometimes tied to capere with a hedge. Standard dictionaries treat the link as probable rather than proven. What is not in doubt is the meaning of the Latin noun: a receptacle, often cylindrical, used for scrolls and other portable goods.
How the two lines kept branching
Once the event-word was in English, its generality invited more jobs. In case, recorded from the mid-fourteenth century, means in the event that. Nineteenth-century compounds such as case law, case history, and case study treat a single instance as the unit of legal, medical, or scholarly attention. Police usage for an incident under investigation is nineteenth-century as well. Mid-nineteenth-century American slang could call a remarkable person a case.
The box-word produced a verb in the late sixteenth century, to enclose in a covering. Early twentieth-century American slang added another verb, to case a place, meaning to look it over, usually before a theft. That sense may rest on inspecting something from all sides, as if tracing the outside of a building. An older technical verb, to case a wall with a different facing, shows how the covering sense could move into action without any criminal coloring.
Printing gives the neatest later split inside the container family. From the 1580s, a case is the compositor’s shallow wooden tray, divided into compartments for letters. Pairing two trays on a sloped stand put capitals higher and farther back, small letters lower and nearer the hand. Upper-case and lower-case for capital and small letters follow from that furniture, with the letter-name itself cited in the nineteenth century. The metaphor is physical, not grammatical. Nothing in that story depends on Latin cāsus.
What the collision hides
English speakers rarely feel the seam. A court case and a violin case share sound and spelling, so they feel related. They are not. One is a fall that became an event, then an instance, then a lawsuit and a grammatical label. The other is a holder, from a Latin box for books, later a suitcase, a casing, and the trays that gave printers their names for capital and small type. If one detail is worth keeping, it is that uppercase letters sit upstairs in the language because compositors once reached toward the higher tray of lead, not because a noun had fallen from the nominative.