Thinness is from thin (Old English thynne) + the abstract-noun suffix -ness, from Proto-Germanic *thunnuz, from the Proto-Indo-European root *ten- ('to stretch'). Important recorded forms include thynne (Old English), *thunnuz (Proto-Germanic), and *ten- (Proto-Indo-European). Morphologically, the native -ness suffix turns an adjective or quality word into an abstract noun naming the state or condition. The modern sense is largely compositional, with the historical elements narrowing into a stable English lexical item. In current use it covers the quality of being thin or slender. The wider family includes related words such as thin, thinly, and thinning.