/koʊˌɪnsɪˈdɛntəl/
[adjective] Happening by chance at the same time or in the same way, without being planned or connected. When two things are coincidental, they seem related but are actually just accidents of timing or circumstance.
The word 'coincidental' literally means 'falling together' — as if two events simply tripped over each other by accident.
To understand 'coincidental', you have to travel back to classical Latin and a verb that described something as simple as falling. The Latin word 'cadere' meant 'to fall', and it sat at the centre of a large family of words that the Romans used to talk about things dropping, happening, or landing in a particular way. From 'cadere' came 'incidere', meaning 'to fall upon' or 'to happen', which in turn produced the noun 'incidentia', meaning an occurrence or event. Add the prefix 'co-' — meaning 'together' — and you get 'coincidere', the idea of two things falling in the same place at the same time.
This Latin root passed into Medieval Latin as 'coincidere', which travelled into French as 'coïncider' in the 16th century. English borrowed the verb 'coincide' directly from French around the 1620s, using it at first in a fairly technical, mathematical sense. Early writers used it to describe geometric figures that occupy exactly the same space — two lines that lie perfectly on top of each other were said to coincide. The idea of chance or surprise was not yet part of the word.
From 'coincide', English speakers formed the noun 'coincidence' in the mid-17th century. At first it still carried that cool, neutral sense of two things occupying the same position or moment. But language is a living thing, and over the following decades 'coincidence' began to warm up emotionally. People started using it to describe those strange, surprising moments when unconnected events mirror each other — the feeling that the universe has arranged something without being asked.
The adjective 'coincidental' arrived a little later, built from 'coincidence' by adding the familiar suffix '-al', which English uses to turn nouns into adjectives (as in 'accident' becoming 'accidental', or 'incident' becoming 'incidental'). Interestingly, 'incident', 'accident', and 'coincidental' are all distant cousins, all tracing back to Latin 'cadere' — 'to fall'. An accident is something that 'falls towards you' (from Latin 'accidere'), an incident is something that 'falls upon' a situation, and a coincidence is when two things 'fall together'.
The word has carried its modern meaning — describing something that happens by chance rather than by design — comfortably since the 18th century. Today it does important emotional and social work, giving us a way to insist that a surprising similarity or timing is purely accidental. 'It's purely coincidental' is a phrase often heard in courtrooms, newsrooms, and everyday conversation alike, where the border between chance and intention matters enormously. The word carries a faint trace of disbelief built into it, as if the speaker is reassuring someone — or perhaps themselves — that the universe did not plan this after all.