c. 1600, a contraction into one word of the prepositional phrase in dede "in fact, in truth, in reality" (early 14c.), from Old English dæd "a doing, act, action, event" (see deed (n.)). As an interjection, 1590s; as an expression of surprise or disgust, 1834. Emphatic form yes (or no) indeedy attested from 1856, American English.
1794, "to write commentary upon," a back-formation from commentator. It unconsciously revived Middle English commentaten "write a commentary, expound a text" (early 15c.). But this oldest sense in English is rare. The main modern meaning "to deliver commentary" (on some public event) is attested from 1939 (implied in commentating). Related: Commentated.
early 15c., rarite, "thinness, porosity, condition of being not dence;" 1550s, "fewness, state of being uncommon," from French rarité and directly from Latin raritas "thinness, looseness of texture; fewness," from rarus (see rare (adj.1)). Sense of "a rare thing or event, thing valued for its scarcity or unusual excellence" is from 1590s.
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.