From Middle English -nes, -nesse, from Old English -nis, -nes, from Proto-Germanic *-nassuz. This suffix was formed already in Proto-Germanic by false division of the final consonant *-n- of the preceding stem + the actual suffix *-assuz. The latter was in turn derived from an earlier *-at(s)-tuz, from the verbal suffix *-at-janą + the noun suffix *-þuz.
- Appended to adjectives to form nouns meaning "the state of being(the adjective)", "the quality of being(the adjective)", or "the measure of being(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.
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.