Yours, Etc.: Origins and Uses of 8 Sign-Offs
early 14c., in dede "in fact, in truth," from Old English dæd (see deed). Written as two words till c.1600. As an interjection, 1590s; as an expression of surprise or disgust, 1834. Emphatic form in yes (or no) indeedy attested from 1856, American English.