Green’s Dictionary of Slang

Spanish onion n.

[rhy. sl.]

a bunion.

[Aus]Sport (Adelaide) 6 July 13/4: They Say [...] That the bell bird—alias Chew-hi-hi, the bunion king—is now trying white ant killer on his old Spanish onion.
[UK]S.T. Kendall Up the Frog.
[UK]R. Puxley Cockney Rabbit.