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accident [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]
accidence (n.): late 14c., in philosophy, "non-essential or incidental characteristic," also "part of grammar dealing with inflection" (mid-15c.), in some cases a misspelling of accidents, or else directly from Latin accidentia (used as a term in grammar by Quintilian), neuter plural of accidens, present participle of accidere (see accident). The grammar sense is because they change in accordance with use.
accident (n.): 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.
accidental (adj.): late 14c., "non-essential," from Old French accidentel or directly from Medieval Latin accidentalis, from Latin accidentem (see accident). Meaning "outside the normal course of nature" is from early 15c.; that of "coming by chance" is from 1570s.
accidental (n.): late 14c., "non-essential quality," from accidental (adj.). The musical sense is from 1868.
accidentally (adv.): late 14c., "non-essentially," also "unnaturally," from accidental + -ly (2). Meaning "unintentionally" is recorded from 1580s; phrase accidentally on purpose is recorded from 1862.
flaccid (adj.): 1610s, from French flaccide or directly from Latin flaccidus "flabby, pendulous, weak, drooping," from flaccus "flabby, flap-eared," which is of uncertain origin (OED suggests it's imitative). Related: Flaccidly; flacidness; flaccidity; flaccescency.
accidently: "In an accidental or chance manner; accidentally, by chance (contextually: by mischance)", Late Middle English; earliest use found in Guy de Chauliac's Grande Chirurgie. Either from accident + -ly, or a variant of accidentally.