Green’s Dictionary of Slang

the making of uniforms, usually military but also others (see cite 1849).

1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1849		Bell’s New Wkly Messenger 16 Dec. 3/2: The journeymen tailors who have not the command of a shop are principally dependent upon [...] ‘sank work’. This consists of soldiers, police, Custom-house, post and mail clothing.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 344/2: My wife works at the ‘sank work.’ She makes soldiers’ coats.
1870		Hotten Sl. Dict.