Accidental
late Middle English (in accidental (sense 2 of the adjective and sense 3 of the adjective)): from late Latin accidentalis, from Latin accident- ‘happening’ (see accident).
wiktionary
From Middle English accidental, from Anglo-Norman accidentel, Middle French accidentel, accidental, and their source, Late Latin accidēntālis; corresponding to accident + -al.
etymonline
accidental (adj.)
late 14c., "non-essential," from Old French accidentel or directly from Medieval Latin accidentalis, from Latin accidentem "an accident, chance" (see accident). Meaning "outside the normal course of nature" is from early 15c.; that of "coming by chance, unintentional" is from 1570s. Accidential (1811) sometimes serves now in the sense "characterized by non-essential qualities" and goes with accidence.
accidental (n.)
late 14c., "non-essential quality," from accidental (adj.). The musical sense is from 1868; so called because they alter the note without essentially changing the key of the passage.
