Alternative forms 
Middle English, from Old English -nis, -nes (“-ness”), from Proto-Germanic *-nassuz (“-ness”), alteration (by false division) of -n- (adjective and verbal terminating consonant) + Proto-Germanic *-assuz (“-ness”). Akin to Old Saxon -nissi, -nussi (Dutch -nis), Old High German -nissa, -nassi, -nussi (German -nis), Gothic -inasus, -inassus
- Appended to adjectives to form nouns meaning "the state of (the adjective)", "the quality of (the adjective)", or "the measure of (the adjective)".
- Appended to words of other parts of speech to form nouns (often nonce words or terms in philosophy) meaning the state/quality/measure of the idea represented by these words.
Usage notes 
If adjective ends in -y, then this changes to -i- when -ness is suffixed. This occurs both when the -y is the suffix -y (“having the quality of”), as in mess → messy → messiness (hence -y → -i-), but also in other cases, as in comely → comeliness.
Derived terms