Etymology
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incident (adj.)

late 15c., "likely to happen," from Latin incidentem (nominative incidens), present participle of incidere "to happen, befall" (see incident (n.)). From 1620s as "occurring as a subordinate;" 1660s in literal sense "falling or striking upon."

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incident (n.)
early 15c., "something which occurs casually in connection with something else," from Old French incident (13c.), and directly from Latin incidentem (nominative incidens), present participle of incidere "to fall in, fall, find the way; light upon, fall in with; fall upon, occur; happen, befall," from in- "on" (from PIE root *en "in") + -cidere, combining form of cadere "to fall" (from PIE root *kad- "to fall"). Broader sense of "an occurrence viewed as a separate circumstance" is from mid-15c. Euphemistic meaning "event that might trigger a crisis or political unrest" first attested 1913.
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happening (adj.)
1520s, "occurring," present-participle adjective from happen (v.). Compare incident.
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incidental (adj.)
"casual, occurring casually in connection with something else; of minor importance," 1640s, from Medieval Latin incidentalis, from incidens (see incident (n.)). The earlier adjective in this sense was incident (1520s). Incidentals (n.) "'occasional' expenses, etc.," is attested by 1707. Incidental music "background music," originally in operas, is from 1812.
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incidence (n.)

early 15c., "incidental matter," from Old French incidence (15c.), from Late Latin incidentia, from incidere "to happen, befall" (see incident (n.)). Meaning "act of coming into contact with or affecting" is from 1650s. In physics, of rays of light, etc., considered with reference to direction, from 1620s.

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*kad- 
Proto-Indo-European root meaning "to fall."

It forms all or part of: accident; cadaver; cadence; caducous; cascade; case (n.1); casual; casualty; casuist; casus belli; chance; cheat; chute (n.1); coincide; decadence; decay; deciduous; escheat; incident; occasion; occident; recidivist.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sad- "to fall down;" Latin casus "a chance, occasion, opportunity; accident, mishap," literally "a falling," cadere "to fall, sink, settle down, decline, perish;" Armenian chacnum "to fall, become low;" perhaps also Middle Irish casar "hail, lightning."
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appurtenant (adj.)
"belonging, incident, or pertaining to," late 14c., from Anglo-French apurtenant, Old French apartenant, apertenant, present participle of apartenir "be related to" (see appurtenance).
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all-nighter (n.)

"incident of staying up all night," 1870, from the adverbial phrase; see all + night. By 1930 as "person who stays up all night."

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blood-curdling (adj.)
also bloodcurdling, 1817, from blood (n.) + present participle of curdle. Also formerly with a noun form, bloodcurdler "incident which freezes the blood," especially "sensational story," 1877, slang; also in use in this sense was blood-freezer (1886).
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relive (v.)

also re-live, 1540s, "to come to life again, revive" (also "to restore to life again, recall to life," a sense now archaic), from re- "back, again" + live (v.). Meaning "to experience (an incident, a period of time) over again" is attested from 1711. Related: Relived; reliving.

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