'apetalous' originates from New/Modern Latin (botanical use), specifically built from the prefix 'a-' meaning 'without' and Greek 'petalon' meaning 'leaf' or 'petal'.

Greek 'petalon' ('leaf, thin plate, petal') gave rise to Latin/Modern Latin forms (via French 'pétale' and English 'petal'), and the adjective 'apetalous' was formed in Neo-Latin/botanical Latin by prefixing 'a-' to this root.

Initially it was a technical botanical formation meaning 'without petals', and it has retained that specific botanical meaning in modern English.