happening (n.)

late 14c., happenen, "to come to pass, occur, come about, be the case," literally "occur by hap, have the (good or bad) fortune (to do, be, etc.);" extension (with verb-formative -n) of the more common hap (v.). Old English used gelimpan, gesceon, and Middle English also had befall. In Middle English fel it hap meant "it happened." Related: Happened; happening. Phrase happens to be as an assertive way to say "is" is from 1707.

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.

1855, from happening + ending from circumstance.