E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Frogs, fenmen. The Mexicans are so called.
When the Queens Prize was won at Wimbledon, July 21st, 1885, by Sergeant Bulmer, 2nd Lincoln, his victory was hailed with Well done, yellow-belly! in allusion to his being a Lincolnshire man.Notes and Queries, August 22nd, 1885, p. 146.
Ah, then, agin, it kin scarce be Mexikins neyther. It ur too fur noth for any o them yellow-bellies.Coptain Mayne Reid: The War Trail, chap. lxxi.