'petalousness' originates from Modern English, specifically formed from the adjective 'petalous' + the suffix '-ness', where 'petalous' is ultimately from New Latin 'petalosus' and Greek 'petalon' meaning 'leaf' or 'thin plate' (used for a leaf of a flower).

'petalon' (Greek) was adopted into New/Modern Latin as 'petalum' or related forms; English borrowed 'petal' (19th century) from these classical sources, which produced the adjective 'petalous' and later the noun formed with '-ness', 'petalousness'.

Initially the root referred to a 'leaf' or 'thin plate' (petalon); over time it came to denote the leaf-like parts of a flower ('petal') and eventually the adjective/nominal sense 'having petals' as in 'petalousness'.