Probably originated from the suffix -ость (-ostʹ) added to words ending in -ный (-nyj), and was then extracted by metanalysis as a productive suffix. The same thing happened with e.g. -те́льный (-télʹnyj) and English -ness.
-ность • (-nostʹ)
- Suffix forming nouns denoting the quality of the action designated by the verbs from which the nouns are formed.
- Suffix forming abstract nouns from adjectives. A less productive alternative to -ость (-ostʹ).
- Suffix forming nouns with the meaning of a group of things characterized by the quality designated by the adjectives from which the nouns are formed.
Not all words in -ность (-nostʹ) contain this suffix. Most of them are formed from -ость (-ostʹ) added to adjectives in -ный (-nyj), e.g. вре́дность (vrédnostʹ, “harmfulness”) from вре́дный (vrédnyj, “harmful”).
- T. F. Yefremova (2006), The New Dictionary of the Russian Language