mid-14c., from honest + -ly (2). As an intensifier or exclamation, from 1898.
Entries linking to honestly
honest adj.
c. 1300, "respectable, decent, of neat appearance," also "free from fraud," from Old French oneste, honeste "virtuous, honorable; decent, respectable" (12c.; Modern French honnête), from Latin honestus "honorable, respected, regarded with honor," figuratively "deserving honor, honorable, respectable," from honos (see honor (n.)) + suffix -tus. Main modern sense of "dealing fairly, truthful, free from deceit" is c. 1400, as is sense of "virtuous, having the virtue of chastity" (of women). Phrase to make an honest woman of "marry (a woman) after seduction" is from 1620s.
-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.