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.
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."