'placidity' originates from Latin, specifically the word 'placiditas', where 'placidus' meant 'calm' and the root 'placare' meant 'to please' or 'to soothe'.

'placidity' entered English via French 'placidité' (modern French) in the 17th century, the French term itself coming from Latin 'placiditas'.

Initially it meant 'calmness or mildness' in the same sense as Latin; over time it has retained that core sense, referring to both physical calm (e.g., a placid sea) and mental calm.