early 15c., "pertaining to the faculty of sight;" also "coming from the eye or sight" (as a beam of light was thought to do), from Late Latin visualis "of sight," from Latin visus "a sight, a looking; power of sight; things seen, appearance," from visus, past participle of videre "to see" (see vision). Meaning "perceptible by sight" is from late 15c; sense of "relating to vision" is first attested c. 1600. The noun meaning "photographic film or other visual display" is first recorded 1944.
-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.