Meaning

adverb in truth (often tends to intensify)

adverb (used as an interjection) an expression of surprise or skepticism or irony etc.

originFrom Middle English indede, univerbation of the phrase in dede (“in sooth, in fact”). Cognate with Saterland Frisian innerdoat, innedoat (“indeed”), West Frisian yndied (“indeed”), Dutch inderdaad (“indeed”), German in der Tat (“indeed”). By surface analysis, in + deed. Compare in fact, in truth, etc. First attested in the early 14ᵗʰ century.

Etymology from Wiktionary, CC BY-SA 3.0