late 12c., mēk, "gentle or mild of temper; forbearing under injury or annoyance; humble, unassuming;" of a woman, "modest," from a Scandinavian source such as Old Norse mjukr "soft, pliant, gentle," from Proto-Germanic *meukaz (source also of Gothic muka-modei "humility," Dutch muik "soft"), a word of uncertain origin, perhaps from PIE *meug- "slippery, slimy." In the Bible, it translates Latin mansuetus from Vulgate (for which see mansuetude). Sense of "submissive, obedient, docile" is from c. 1300.
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.