Entries linking to surely
As an affirmative meaning "yes, certainly" it dates from 1803, from Middle English meanings "firmly established; having no doubt," and phrases like to be sure (1650s), sure enough (1540s), and for sure (1580s). The use as an adverb meaning "assuredly" goes back to early 14c. Sure-footed is from 1630s, literal and figurative; sure thing dates from 1836. In 16c.-17c., Suresby was an appellation for a person to be depended upon (see rudesby).
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.