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.
