late 14c., soundnes, "solidity, firmness," from sound (adj.) + -ness. Of the mind, etc. by 1540s.
Entries linking to soundness
sound adj.
"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.
-ness
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-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).