'ballgown' originates from English, formed by combining the words 'ball' and 'gown', where 'ball' meant 'a formal social dance' and 'gown' meant 'a woman's long dress'.

'ball' changed from Old French 'bal' (from Medieval Latin/Latin roots related to dancing) and entered Middle English as 'bal'; 'gown' changed from Old French 'goune' and Middle English 'goun' to the modern English 'gown'.

Initially it meant 'a dress for a ball', and over time it has retained that meaning as 'a woman's formal dress for a ball or very formal event'.