c. 1300, "terrifying, horrible, dreadful," from Anglo-French hidous, Old French hideus, earlier hisdos "hideous, horrible, awful, frightening" (11c.; Modern French hideux), from hisda "horror, fear," perhaps of Germanic origin. The old guess that it comes from Vulgar Latin *hispidosus, from Latin hispidus "shaggy, bristly," "presents numerous difficulties" [OED] and seems now to be generally discredited. Meaning "repulsive" is late 14c.
-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.