'serenidad' originates from Latin, specifically the word 'serenitas', where 'serenus' meant 'clear' or 'calm'.

'serenitas' passed into Old Spanish as 'serenidad' (via Vulgar Latin influences) and eventually became the modern Spanish word 'serenidad'.

Initially, it meant 'clearness' or 'calmness' (often of the sky), and over time it broadened to its current meaning of 'serenity' or 'mental/atmospheric tranquility'.