'petalless' originates in English as the combination of the noun 'petal' and the suffix '-less', where 'petal' comes from French 'pétale' (from Greek 'petalon') and '-less' is an Old English/Germanic suffix meaning 'without'.

'petal' was borrowed into modern European languages from Greek 'petalon' meaning 'leaf' or 'thin plate'; it entered English (via French/Latin influence) in the late 18th/early 19th century as the technical botanical term 'petal'. The adjective 'petalless' was formed later in English by adding the productive suffix '-less' to mean 'without petals'.

Initially 'petalon' in Greek meant 'leaf' or 'thin plate'; over time in modern languages 'petal' came to mean the colored segments of a flower, and 'petalless' straightforwardly came to mean 'without those petals'.