'skinniness' originates from Modern English, specifically formed from the adjective 'skinny' plus the noun-forming suffix '-ness'. 'Skinny' itself is derived from 'skin' with the adjectival suffix '-y'.

'skin' comes from Old Norse 'skinn' meaning 'hide' or 'pelt'; English formed 'skin' from that source. The adjective 'skinny' developed in English (skin + -y) to describe someone with little flesh; later the abstract noun 'skinniness' was formed by adding '-ness'.

Initially related to 'skin' and the notion of hide or surface, the adjective 'skinny' came to mean 'having little flesh or fat', and 'skinniness' retained that sense of thinness; over time it also gained figurative uses (e.g. scantiness or lack of substance).