'solidez' originates from Latin, specifically the word 'soliditas', where 'solidus' meant 'solid'.

'solidez' changed from Late Latin 'soliditas' → Old Spanish/Old Castilian forms such as 'solideç' (medieval) and eventually became the modern Spanish word 'solidez'.

Initially, it meant 'the state or quality of being solid', and over time it broadened to include 'stability' or 'soundness' in abstract contexts (e.g., financial or logical soundness).