Green’s Dictionary of Slang

to borrow money; thus the rip-rap n., the act of obtaining such a loan.

1946		S. Jackson Indiscreet Guide to Soho 32: It is easy enough to talk to anybody in Soho once people are sure that you are not on the ‘rip-rap’ (‘tap’ or scrounge).
1950		P. Hoskins No Hiding Place! 191/1: On the Rip-rap. Cadging.
1960		Kerouac letter 20 May in Charters II (1999) 254: I have riprap, etc. already.
1981		J. Franklyn Dict. Rhy. Sl. (2nd edn).