One of the mysteries of the English language finally explained.
Having an orange belly. Used chiefly in the names of (frequently Australian) animals and birds, especially "orange-bellied parrot"noun a rare parrot with a predominantly green back and wings, Neophema chrysogaster, which is native to western Tasmania but migrates to coastal areas of western Victoria in Australia (formerly called "orange-bellied (grass-)parakeet").
Mid 19th century; earliest use found in John Gould (1804–1881), ornithologist and publisher. From orange + bellied.
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