'solidness' originates from English, formed by the adjective 'solid' plus the suffix '-ness'. The adjective 'solid' ultimately comes from Latin 'solidus', meaning 'firm, whole'.

'solid' changed from Latin 'solidus' to Old French 'solide' and then to Middle English 'solid'; the modern English noun 'solidness' developed by adding the productive English suffix '-ness' to the adjective.

Initially, the root 'solidus' meant 'firm, whole'; over time this produced the English adjective 'solid' with a similar sense, and 'solidness' came to mean 'the state or quality of being solid' or 'the firmness/density of something'.