indeedy has the recorded history: A colloquial, chiefly American emphatic extension of indeed (in + deed) with a playful -y suffix, popularized in the 19th century. Morphologically, the ending is part of the etymology: the suffix is attached to the older base named in the history, producing a later English derivative. In present English, the adverb sense is "indeed (a colloquial, emphatic, often jocular intensifier)", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as indeed and deed. Its semantic neighborhood includes indeed, certainly, absolutely, and truly, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English dǣd and Proto-Indo-European *dʰeh₁-.