1540s, "in an unfortunate manner, by ill-fortune," from unfortunate + -ly (2). The original meaning is now rare; the main modern sense of "sad to say, unhappily, unluckily," in parenthetical use, is recorded from 1770s.
mid-15c., "unlucky," from un- (1) "not" + fortunate (adj.). Infortunate in same sense is older. In late 18c.-early 19c., unfortunate woman was a polite way to say "prostitute." The noun meaning "one who is not fortunate" is recorded from 1630s.
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.
<a href="https://www.etymonline.com/word/unfortunately">Etymology of unfortunately by etymonline</a>
Harper, D. (n.d.). Etymology of unfortunately. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/unfortunately