Casual
late Middle English (in casual (sense 2 of the adjective, sense 3 of the adjective)): from Old French casuel and Latin casualis, from casus ‘fall’ (compare with case1).
wiktionary
From Middle French casuel, from Late Latin cāsuālis(“happening by chance”), from Latin cāsus(“event”) (English case), from cadere(“to fall”) (whence English cadence).
etymonline
casual (adj.)
late 14c., casuel, "subject to or produced by chance," from Old French casuel (15c.), from Late Latin casualis "by chance," from Latin casus "chance, occasion, opportunity; accident, event" (see case (n.1)).
Of persons, in the sense of "not to be depended on, unmethodical," it is attested from 1883 (from the notion of "without regularity," hence "uncertain, unpredictable"); meaning "showing lack of interest" is from 1916. Of clothes, "informal," from 1891. Related: Casually.
