Green’s Dictionary of Slang

a £5 note.

1847		G.M.W. Reynolds Mysteries of London III 66/1: The skin had three finnips and a foont [...] A fly kidden-gonnoff will leave this flim .
1870		Chambers’s Journal 9 July 448: What would it be worth? A flim, Sam [F&H].
1914		E. Wittmann ‘Clipped Words’ in DN IV:ii 119: flim, from flimsy. A banknote.
1926		Portsmouth Eve. News 23 Nov. 4/4: A man with [...] only £1 or £2 is a ‘case phunt’ or a ‘deuce phunt’. If he has £5 he is a ‘flim’. £10 an ‘Uncle Ben’, and more than that a ‘John Peel’ or a ‘Sarker’.
1934		P. Allingham Cheapjack 38: A fiver is a ‘flim’.
1943		M. Harrison Reported Safe Arrival 25: There’s nothin’ to do ’cep go to ’Arringay and shove a flim on the bow-wows.
1956		‘Charles Raven’ Und. Nights 90: You can give me a flim for the introduction.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 119: ‘A jacks.’ ‘A jacks?’ ‘Like a flim.’ ‘A flim?’ [...] ‘A handful,’ he said. ‘Oh — five pounds?’.
1989		J. Morton Lowspeak.
2001		K. Sampson Outlaws (ms.) 66: He’d’ve just thought I was pallatic blacking out in his cab like that, giving him two and a half pound out of a flim.