Indeed

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Middle English: originally as in deed .

wiktionary

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From Middle English indede, contraction of the phrase in dede(“in sooth, in fact”)[early 14thc.]; equivalent to in +‎ deed (similar in formation to in fact, in truth, etc.). Cognate with Saterland Frisian innerdoat, innedoat(“indeed”), West Frisian yndied(“indeed”), Dutch inderdaad(“indeed”), German in der Tat(“indeed”).

etymonline

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indeed (adv.)

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.