An Etymological Dictionary of Astronomy and Astrophysics

A native English suffix attached to adjectives and participles, forming abstract nouns denoting quality and state, such as brightness, cloudiness, compactness, skewness, and so on.

See also: M.E., O.E. -nes(s), cf. M.Du. -nisse, O.H.G. -nissa, Ger. -niss.