- -ness suffix denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from O.E. -nes(s), from West Germanic *in-assu- (Cf. O.S. -nissi, M.Du. -nisse, Du. -nis, O.H.G. -nissa, Ger. -nis, Goth. -inassus), from *-in-, noun stem, + *-assu-, abstract noun suffix, probably from the same root as L. -tudo (see -TUDE (Cf. -tude)).
Etymology dictionary. 2014.