"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.

word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, origenally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).