'equal-sepalled' originates from modern English, formed from the adjective 'equal' (ultimately from Latin 'aequalis') combined with 'sepal' (from New Latin 'sepalum').

'sepal' comes from New Latin 'sepalum', a 19th-century botanical formation influenced by Greek 'skepē' meaning 'covering'; the compound adjective 'equal-sepalled' is a later English formation combining 'equal' + 'sepal' + adjectival '-ed'.

Initially it meant 'having sepals of equal size', and this specific botanical meaning has been retained in modern usage.