accidentyoudaoicibaDictYouDict[accident 词源字典]
accident: [14] Etymologically, an accident is simply ‘something which happens’ – ‘an event’. That was what the word originally meant in English, and it was only subsequently that the senses ‘something which happens by chance’ and ‘mishap’ developed. It comes from the Latin verb cadere ‘fall’ (also the source of such diverse English words as case, decadent, and deciduous).

The addition of the prefix ad- ‘to’ produced accidere, literally ‘fall to’, hence ‘happen to’. Its present participle was used as an adjective in the Latin phrase rēs accidēns ‘thing happening’, and accidēns soon took on the role of a noun on its own, passing (in its stem form accident-) into Old French and thence into English.
=> case, decadent, deciduous[accident etymology, accident origin, 英语词源]
accident (n.)youdaoicibaDictYouDict
late 14c., "an occurrence, incident, event," from Old French accident (12c.), from Latin accidentem (nominative accidens), present participle of accidere "happen, fall out, fall upon," from ad- "to" (see ad-) + cadere "fall" (see case (n.1)). Meaning grew from "something that happens, an event," to "something that happens by chance," then "mishap." Philosophical sense "non-essential characteristic of a thing" is late 14c. Meaning "unplanned child" is attested by 1932.