'sepalous' originates from Neo-Latin 'sepalum' plus the English adjectival suffix '-ous', where 'sepalum' ultimately comes from Greek 'skēpē' meaning 'covering'.

'sepalous' developed from Neo-Latin 'sepalum' (a modern Latin formation from the 19th century) into English botanical usage as 'sepal' and then formed as the adjective 'sepalous' by adding '-ous'.

Initially it referred to the concept of a 'covering' (from Greek) as applied to the leaflike parts of a flower; it has kept this sense and now specifically means 'having or relating to sepals'.