1845, "timidly," from scary + -ly (2). By 1967 in a positive sense, "unnervingly" (as in scarily good, etc.).
Entries linking to scarily
scary adj.
also scarey, "terrifying, causing or tending to cause fright," 1580s, from scare (n.) + -y (2). Meaning "easily frightened, subject to scares" is from 1800. In this sense formerly sometimes colloquially as skeery, skeary; OED marks this meaning as originally and chiefly North American. Related: Scarier; scariest.
-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.