What speakers mean by it now
In ordinary English, accident names an unplanned event, usually one that causes harm or at least inconvenience: a collision, a fall that breaks a bone, a meeting that nobody arranged. A second, more abstract sense is chance itself, as in meeting by accident rather than by design. A third sense, still current in philosophy and logic, is a quality a thing happens to have rather than a quality that makes it what it is.
Those uses look unrelated until the older picture is restored. The word began as a name for something that befalls a person or a situation, not something aimed at it.
How English took the word
English first records the noun in the late fourteenth century. It came through Old French accident, already in French in the twelfth century, and through Latin accidentem, the accusative of accidens. Standard historical dictionaries treat the English form as having more than one immediate source at once: partly French, partly Latin. That double intake is common for learned vocabulary in the later Middle Ages, when the same shape could appear in a vernacular text and in a Latin treatise.
Latin accidens is the present participle of accidere, to happen, fall out, or fall upon. The verb is built from ad, toward, and cadere, to fall. The image is physical before it is moral: an event that drops onto the scene. English already had native words for mishaps. Accident entered as a borrowed technical term, not as everyday slang for a crash.
In Middle English the noun is met most often in three specialist registers. Theology used it for the outward qualities of bread and wine in the Eucharist after the change of substance. Medicine used it for something out of the ordinary in the body: a symptom, a disease, an injury. Philosophy used it for a non-essential characteristic, following Latin accidens as a rendering of the Aristotelian term for a contingent attribute as opposed to substance. One major dictionary even dates that philosophical sense as the first known English use in the fourteenth century, with the everyday unplanned event listed later. The historical order of senses in English does not line up neatly with the logical chain from anything that happens to mishap.
A verb of falling, then a noun of happening
The Latin verb cadere belongs to a family English has borrowed many times. Cadence, cascade, decay, cadaver, chance, case in the legal and grammatical senses that come from Latin casus (a falling), incident, coincide, and occasion all rest on the same motion. The deeper root is reconstructed as Proto-Indo-European *kad- or *ḱad-, to fall. Close verbal relatives of cadere include Spanish caer and Italian cadere.
The morphology of accident is therefore transparent once the parts are named: the prefix ad-, the stem of cadere, then the participle ending that turned a verb of happening into a noun of happening. Stories that treat the word as if it had always meant a road crash miss that structure. The fall in the etymology is grammatical history, not a description of every modern mishap.
French accident, Spanish accidente, Italian accidente, and Portuguese acidente are close siblings, all from the same Latin participle. German Unfall is a different construction, un- plus Fall, a fall: a semantic parallel, not a loan of the Latin form. Dutch has used a borrowed accident beside native words for mishaps.
From occurrence to misfortune
Already in Latin the participle could slide from a mere occurrence toward misfortune. A formula of the type if anything should happen to someone could stand as a polite way of talking about death. That euphemistic pressure never quite left the word.
By the late fifteenth century English could use accident for the operations of chance. The everyday sense of an unplanned, usually unwelcome event grew from there and eventually crowded out the older, more neutral incident. Medical and legal English kept a technical edge. A cerebrovascular accident is still a stroke in clinical language. In law an accident can be an unexpected happening that causes loss, sometimes even when the trigger was not a simple slip.
Colloquial English added further layers later. Accident-prone is recorded from 1926. By 1932 the noun could name an unplanned child. A household euphemism uses accident for an involuntary call of nature, especially of a baby or a pet. Those are not medieval meanings; they are later specializations of the same idea of something that was not intended.
One obsolete path is worth noting because it shows how the philosophical sense once leaked into grammar. Latin accidentia could mean the variable features of words: gender, number, case, mood, tense. English briefly had that use, then replaced it with accidence, the old schoolroom name for inflection. The two spellings are easy to confuse; they are historically neighbors.
What the falling root still explains
The modern traffic crash is a late, vivid specialization. The origin of the word accident is older and drier: a Latin participle for what falls toward you, borrowed into English with theology, medicine, and logic already attached. The most durable leftover of that history is not the crash, but the contrast still used in philosophy: substance versus accident, what a thing is versus what merely happens to it.