'petalous' originates from Greek and New Latin, specifically the Greek word 'petalon', where 'petalon' meant 'leaf' or 'thin plate' (used for petals). The adjective was formed in English by adding the suffix '-ous' to 'petal'.

'petalon' changed into Neo-Latin/Latin 'petalum', then into French 'pétale' and English 'petal'; the modern English adjective 'petalous' was formed from 'petal' + '-ous'.

Initially it referred to a 'leaf' or 'thin plate' (the original Greek sense), but over time it came to mean specifically 'having petals' in botanical contexts.