'assiduousness' originates from Latin, specifically the word 'assiduus', where 'ad-' meant 'to' and 'sedere' meant 'to sit'.

'assiduous' came into English from Late Latin 'assiduus' (via Medieval/Old French and Middle English forms) and developed into the modern English noun 'assiduousness'.

Initially it meant 'continually present or attending' in Latin; over time it evolved into the modern meaning of 'persistent, careful attention or diligent application'.