Green’s Dictionary of Slang

calves gone to grass phr.

[a pun]

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.

[UK]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.
[UK]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.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]‘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’.