incidental (adj.)

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.

1520s, "by the way, casually;" see incidental + -ly (2). Sense of "as a new but related point" attested by 1925.