Old English wid "vast, broad, long," also used of time, from Proto-Germanic *widaz (source also of Old Saxon, Old Frisian wid, Old Norse viðr, Dutch wijd, Old High German wit, German weit), perhaps from PIE *wi-ito-, from root *wi- "apart, away, in half."
Meaning "distended, expanded, spread apart" is from c. 1500; sense of "embracing many subjects" is from 1530s; meaning "missing the intended target" is from 1580s. As a second element in compounds (such as nationwide, worldwide) meaning "extending through the whole of," is is from late Old English. As an adverb, Old English wide. Wide open "unguarded, exposed to attack" (1915) originally was in boxing, etc. Wide awake (adj.) is first recorded 1818; figurative sense of "alert, knowing" is attested from 1833.
-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.