1865 (in an Artemus Ward dialect humor piece), from thus + -ly (2). A double adverb. Perhaps originally a humorous or mocking over-correction of thus; it has gained some currency but earns frowns for the user.
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.
<a href="https://www.etymonline.com/word/thusly">Etymology of thusly by etymonline</a>
Harper, D. (n.d.). Etymology of thusly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/thusly