"of yore, in times long since past," c. 1500, from ancient (adj.) + -ly (2).
Entries linking to anciently
ancient adj.
late 14c., auncyen, of persons, "very old;" c. 1400, of things, "having lasted from a remote period," from Old French ancien "old, long-standing, ancient," from Vulgar Latin *anteanus, literally "from before," adjectivization of Latin ante "before, in front of, against" (from PIE *anti "against," locative singular of root *ant- "front, forehead"). The unetymological -t dates from 15c. by influence of words in -ent.
From early 15c. as "existing or occurring in times long past." Specifically, in history, "belonging to the period before the fall of the Western Roman Empire" (c. 1600, contrasted with medieval and modern). In English law, "from before the Norman Conquest."
As a noun, "very old person," late 14c.; "one who lived in former ages," 1530s. Ancient of Days "supreme being" is from Daniel vii.9. Related: Anciently.
-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.