-ність

Probably originated from the suffix -ість (-istʹ) added to words ending in -ний (-nyj), and was then extracted by rebracketing as a productive suffix. The same thing happened with e.g. -овий (-ovyj) and English -ness. From -ний (-nyj) +‎ -ість (-istʹ). Compare Russian -ность (-nostʹ), Belarusian -насць (-nascʹ).

-ність • (-nistʹ)

1. Suffix forming abstract nouns from adjectives. A less productive alternative to -ість (-istʹ), except after ч, where it is the regular suffix.

   гото́вий (hotóvyj, “ready”) → гото́вність (gotóvnistʹ, “readiness”) (no such word *гото́вний exists, making it a special case.)

2. 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.

   живи́й (žyvýj, “living”) → жи́вність (žývnistʹ, “small creatures collectively”)

3. Suffix forming nouns of various sorts from miscellaneous parts of speech.

   суть (sutʹ, “essence”) → сутність (sutnistʹ, “essenceness”)

• Not all words in -ність (-nistʹ) contain this suffix. Most of them are formed from -ість (-istʹ) added to adjectives in -ний (-nyj).
• As noted above, -ність (-nistʹ) is the regular alternative to -ість (-istʹ) after adjectives whose stem ends in ч, e.g. жіночій (žinočij).

• -щи́на (-ščína), -ство (-stvo), other suffixes for states of being, or being collective'