"in a medial or central position," 1804, from medial (adj.) + -ly (2).
Entries linking to medially
medial adj.
1560s, "pertaining to a mathematical mean," from Late Latin medialis "of the middle," from Latin medius "in the middle, between; from the middle," as a noun (medium) "the middle;" from PIE root *medhyo- "middle." Meaning "occupying a middle position, existing between two extremities or extremes" is attested from 1721.
-ly 2
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.