Green’s Dictionary of Slang

1. a half.

1859		Hotten Dict. of Modern Sl. etc. 127: FLATCH, a half, or halfpenny.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1873		Sl. Dict.
2001		J. Burkardt ‘Back Sl.’ Wordplay 🌐 flatch: half.

2. a halfpenny.

1859		see sense 1.
c.1864		A. Stephens ‘The Chickaleary Cove’ 🎵 And a doesn’t care a flatch, / So long as I’ve a tach, / Some pannum in my Chest – and a tog on!
1873		Sl. Dict.
1881		Morn. Bulletin (Rockhampton, Qld) 18 July 2/6: A half-penny [...] may find the following; ‘bawbees,’ ‘browns,’ ‘camden town,’ ‘coppers,’ ‘ flatch,’ ‘gray,’ ‘madge.’ ‘make,’ ‘mag or maga,’ ‘posh,’ and ‘rap’.
1890		Northampton Mercury 12 Apr. n.p.: ‘Yennep,’ a penny [...] ‘flatch,’ a halfpenny.
1902		Worcs. Chron. 3 May 3/1: The rhyming slang for bread is ‘Lump o’ Lead’. The back-slang is ‘Da-erb’. I heard a man call out to another yesterday that his Rekab had put his da-erb up to owt yannep flatch. Flatch is back slang for halfpenny.
1929		Advertiser (Adelaide) 25 Oct. 32/7: Birmingham tramps and beggars now have their own newspaper. Its title is the ‘Abraham-man’s News.’ [...] The price is one flach.
1935		R.T. Hopkins Life and Death at the Old Bailey 65: Costermongers invariably use the following terms in discussing money transactions [...] Flatch – Halfpenny.
1953		J. Franklyn Cockney 297: Three-halfpence becomes yenep-flatch, and this translated into ordinary speech: the Cockney rarely refers to three-hapence – his idiom is ‘penny-ha-penny’.