1570s, "state or quality of being suitable," from fit (adj.) + -ness. Meaning "state of being physically fit" is from 1935.
Entries linking to fitness
fit adj.
"suited to the circumstances, proper," mid-15c., of unknown origin, perhaps from Middle English noun fit "an adversary of equal power" (mid-13c.), which is perhaps connected to fit (n.1). In athletics, "in condition, properly trained for action," from 1869. Related: Fitter; fittest. Survival of the fittest (1867) coined by H. Spencer.
-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).