/ˌɪnsɪˈdɛntəl/
[adjective] Happening as a minor part of something more important, or by chance rather than as the main purpose. Something incidental is not the main focus but occurs alongside it.
Every 'incidental' thing that happens to you is literally something that 'falls upon' you — from Latin's word for falling.
The story of 'incidental' begins with a remarkably physical Latin verb: *cadere*, meaning 'to fall.' This verb was one of the most productive roots in all of Latin, giving rise to dozens of words that carried the idea of falling — things dropping, events tumbling into existence, moments collapsing into one another. It is a root so ancient and fundamental that it spread through nearly every corner of the English vocabulary, often in disguise.
From *cadere*, Latin formed the noun *casus*, meaning 'a falling' or 'a chance event' — something that simply drops into your life uninvited. More directly relevant to our word, Latin also produced *incidere*, a compound built from the prefix *in-* ('upon' or 'into') and *cadere* ('to fall'), giving the combined sense of 'to fall upon' or 'to happen upon.' From *incidere* came the noun *incidentia*, and then the Medieval Latin adjective *incidentalis*, meaning 'liable to happen' or 'occurring by chance.' The metaphor is wonderfully concrete: an incidental thing is one that simply falls upon a situation from outside, landing there without being planned or invited.
The word entered English in the early 17th century, arriving through Medieval Latin and influenced by the French *incident*, which had already made the journey from Latin *incidentem* into common use in French before English borrowed the concept. English speakers adopted 'incidental' to describe things that occur alongside a main event — not the point of the matter, but present nonetheless. Early uses in English often appeared in legal and philosophical writing, where thinkers needed precise language to distinguish what was essential from what was merely accompanying.
Over the following centuries, the word broadened naturally into everyday speech. 'Incidental expenses,' for instance, became a common phrase in commerce and travel writing, referring to small costs that arise unexpectedly on the side of a larger budget. 'Incidental music' — the background music composed to accompany a play or film rather than stand on its own — became an important term in the arts, showing how 'incidental' had come to describe anything that supports or surrounds a main thing without being the main thing itself.
What makes this etymology quietly wonderful is how many familiar English words share the same falling root. Latin *cadere* → Old French *cheoir* → Middle English gave us 'chance.' Latin *cadere* → Latin *casus* → Old French *cas* → Middle English gave us 'case.' Latin *cadere* → Latin *accidere* ('to fall toward') → Old French *accident* → English gave us 'accident.' Latin *cadere* → Latin *occidere* ('to fall down, to set like the sun') → English gave us 'occident,' meaning the west, where the sun falls. Even 'decay' and 'cadence' belong to this family. So when you describe something as incidental, you are reaching back to an ancient Roman watching something fall — and finding in that tumbling motion a perfect metaphor for the unplanned, the unexpected, and the beside-the-point.