Green’s Dictionary of Slang

a £5 note; thus cross-finnep n., a forged note; note double fin n.

1839		W.A. Miles Poverty, Mendicity and Crime; Report 112: If he finds any ‘finnips’ (5l. notes) in the skin or purse, he gives them to Nelson.
1847		G.W.M. Reynolds Mysteries of London III 85/1: A Stranger—looked like a crocus. To smash three double finnips .
1857		‘Ducange Anglicus’ Vulgar Tongue 38: He sent the yack to church and got three finnips and a cooter for the sawney.
1857		J. Archbold Magistrate’s Assistant 444: Five-pound notes, finnips, ten-pound notes, double finnips.
c.1862		‘Flash Lang.’ in Snowden’s Police Officers [and] Constables Guide 114: I took them to a swag chovey bloak and got 6 finnips and a cooter for the yacks.
1881		Newcastle Courant 2 Sept. 6/5: In his early life he had been a clerk [but] having ‘nicked a finnop’ [...] he was obliged to ‘tip the double’.
1881		Sydney Punch 1 Oct. 7/2: [He] fences the swag for as much as a finnup and three or four casers.
1882		Sydney Sl. Dict. 10/1: Jim cracked a case last night and fenced the swag for ten cooter. He told me as Nel Starlight had flimped a thimble from a lushy bloak who had been to the ball, and fenced it at Mother; S– ’s for a finnip.
1889		Clarkson & Richardson Police! 321: Bad or forged notes ... Spondulics, flimsies, cross-finneps.
1904		Sun (NY) 10 July 29/4: Here is a genuine letter written in thieves’ slang, recently found by the English police [...] They made owt finups, a couter, a red jack, and jerry, and a red spark-prop.
1908		Denton (MD) Journal 7 Mar. 3/5: ‘What shall we say now — a finnup?’ ‘F’what’zs that, sor,’ sez I. ‘Oh,’ sez he, ‘I s’pose you’re a new hand. Five quid – onderstand that?’.
1919		Smith’s Wkly (Sydney) 7 June 9/6: Slang of Money [...] Specific sums are variously named. £500 is a ‘monkey’; £25 a: ‘pony’; £10, a ‘double finnup’; £5, a ‘single finnup’ (word probably a Yiddish form of the Geman ‘funf’) .
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 4: Finnip:£5 note.