Green’s Dictionary of Slang

a mocking cry aimed at one who has very thin legs.

1678		J. Ray Proverbs (2nd edn) 83: Veal will be cheap: Calves fall. A jeer for those who lose the calves of their legs by &c.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: His calves are gone to grass; a saying of a man with slender legs without calves. Veal will be cheap, calves fall; said of a man whose calves fall away.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1890–1904		Farmer & Henley Sl. and Its Analogues.