soundly adv.

"healthy, not diseased, free from special defect or injury," c. 1200, sounde, from Old English gesund "sound, safe, having the organs and faculties complete and in perfect action," from Proto-Germanic *sunda-, from Germanic root *swen-to- "healthy, strong" (source also of Old Saxon gisund, Old Frisian sund, Dutch gezond, Old High German gisunt, German gesund "healthy," as in the post-sneezing interjection gesundheit; also Old English swið "strong," Gothic swinþs "strong," German geschwind "fast, quick"). The German words have connections in Indo-Iranian and Balto-Slavic.

Paired alliteratively with safe (adj.) at least from c. 1300. The meaning "right, correct, free from error, in accord with facts" is from mid-15c. The sense of "holding accepted opinions" is from 1520s. Of arguments, etc., "without logical flaw," 1570s. The meaning "financially solid or safe" is attested from c. 1600; of sleep, "undisturbed, unbroken," from 1540s.

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.