The story
Chinaman + fish: the fish is relatively harmless in Chinese waters, but frequently toxic in Australia. Perhaps Chinese immigrants were noted for eating it when other Australians wouldn't.
Etymology from Wiktionary, CC BY-SA 4.0.
Chinamanfish
Today it means: A type of snapper native to the western Pacific, Symphorus nematophorus, which is a commercial food fish in some areas, but ciguatoxic in others. Full definition →