A lot vs. Alot: 9 Grammatical Pitfalls
suffix forming adjectives from nouns and meaning "having qualities of, appropriate to, fitting;" irregularly descended from Old English -lic, from Proto-Germanic *-liko- (cf. Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (cf. Old English lich "corpse, body;" see lich, which is a cognate; cf. also like (adj.), with which it is identical).
adverbial suffix, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cf. 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" (e.g. French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.