Green’s Dictionary of Slang

filthy, the n.

money.

1869		Sportsman (London) 1 Sept. 2/1: Notes on News [...] If he doesn’t gel ‘the filthy’ direct—which, by the way, is short for filthy lucre—he gets a large price for his cat [...] but he prefers ‘the filthy’ direct.
1905		Bluefield Daily Tel. (WV) 11 Mar. 4/2: In addition [...] the following [names for money] are given: Soap, Long Green, Stuff, Duff, Dust, Wherewith, Plunks Grease, Mejum, Glue, Root, Toadskin, Shiners, Skads, Samoleons, Bones, Spon, Filthy, Needful, Rhino, Shink, Salt, Moppus, Blunt, Dirt, Means, Tar, Ready, Stivers, Hefty Genuine, Dornes, Gilt, Desirable, Flimsy, Nuggets, Cadewy, Wampum.
1945		F. Fay How To Be Poor 20: I put away chunks and gobs of the ‘filthy’ for the ‘rainy days’.