"earlier in time," mid-12c., comparative of forme "first, earliest in time or order," from Old English forma "first," from Proto-Germanic *fruma-, *furma-, from PIE *pre-mo-, suffixed (superlative) form of root *per- (1) "forward," hence "in front of, before, first." Probably patterned on formest (see foremost); it is an unusual case of a comparative formed from a superlative (the Old English -m is a superlative suffix). As "first of two," 1580s.
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.
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Harper, D. (n.d.). Etymology of formerly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/formerly