Green’s Dictionary of Slang

denoting someone who has noticeably thin legs; thus the joc. remark: ‘veal will be cheap, calves fall’ on noticing a man whose calves fall away.

1715		‘Nickydemus Ninnyhammer’ Homer in a nut-shell 34: ’Tis time they send their Calves to Grass, / And live no longer at this pass. / They soon will want Boots that are fit / Upon their Cat-ftick Legs to set.
c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Calves. his Calves are Gone to Grass saying of a man with Slender Legs, without Calves.
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.
1825		Pierce Egan’s Life in London 17 July 197/1: [H]is pins, in the vulgar phrase of the ring, had gone to grass.
1838		‘Alfred Crowquill’ Seymour’s Humourous Sketches (1866) 81: His legs, indeed, became so slight, that many of his jocose companions amused themselves with striking at them with straws as he passed [...] ‘Whoy, Giles!’ remarked one of them, ‘thee calves ha' gone to grass, lad’.