Green’s Dictionary of Slang

paper money.

1920		Ade Hand-made Fables 159: No matter how much of the Crisp a Fellow handles, he feels like a Lazarus if he cannot acquire all of the desirable Items that he seems to think he wants.
1923		Wodehouse Leave it to Psmith (1993) 522: Two thousand of the crispiest.
1937–84		Partridge DSUE (1984) 270/1: from ca. 1850.
1989		(con. 1950s–60s) in G. Tremlett Little Legs 37: All the nice music, the crispies, went under the mattress.