Green’s Dictionary of Slang

a watch; used as object of phr. indicating the theft of a watch.

a.1790		H.T. Potter New Dict. Cant (1795) n.p.: yack and onions watch and seals.
1809		G. Andrewes Dict. Sl. and Cant.
1812		Vaux Vocab. of the Flash Lang.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1839		H. Brandon Dict. of the Flash or Cant Lang. 166: Yack – a watch.
1847		G.W.M. Reynolds Mysteries of London III 85/1: Three wedge-feeders, a yack, and a dee [...] No. 53. Yack and onions .
1848		Flash Dict. in Sinks of London Laid Open 130: Yace [sic] and onions, watch and seal.
1850		Western Times 9 Feb. 6/3: Policeman Guppy [...] heard a conversation between the two prisoners [...] ‘What did the ‘splodger’ (fellow) say; did he find the ‘yack’ (watch) ‘blewed‘ (got rid of).
1857		‘Ducange Anglicus’ Vulgar Tongue 38: He told me as Bill had flimped a yack and pinched a swell of a fawney.
1857		J. Archbold Magistrate’s Assistant (3rd edn) 444: To alter the maker’s name in a watch – to christen a yack.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 51/2: At last he was ‘bowl’d out’ in the very act of ‘nailing a yack’ (stealing a watch).
1871		Bradford Obs. 27 Apr. 6/6: He covered the other chap artful, and nabbed your yack!
1882		Sydney Sl. Dict. 9/2: Dick’s a broker, and has gone out snowdropping, and Chumpy is trying to fence a yack to a muff, or to play a skin game. Dick’s hard up, and has gone out to steal clean clothes (from clotheslines and hedges), and Chumpy is trying to sell a watch to a simple one, or to skin him at cards.
1887		W.E. Henley ‘Villon’s Straight Tip’ in Farmer Musa Pedestris (1896) 176: Suppose you screeve, or go cheap-jack? [...] Or thimble-rig? or knap a yack? / Or pitch a snide? or smash a rag?
1895		Crowe Aus. Sl. Dict. 97: Yack, a watch.
1901		Argus (Melbourne) 20 Sept. 6/4: The watch is otherwise known as a yack, a tail block, or with chain attached as a block and tackle, while the man who has a weakness for watches [...] is called a thimble twister.
a.1909		press cutting in J. Ware Passing Eng. of the Victorian Era 129/2: Put on a fiddle-face and jaw to him about his future, and it’s most likely he and his mates will slosh your mug for you and sneak your yack.