My slang dictionary offered no help, but the Etymological Dictionary of the English Language, WW Skeat, touches on it:

RIP, to divide by tearing open, cut open, tear open for searching into. (Scand.) "Rip up griefe;" Spencser, F. Q. i. 7. 39. [It does not seem to be the same word as ME. rippen, used in the Ormulum in the sense of "seize;" this is a variant of ME. ruppen, to rob, Layamon, 10584, and allied rather to G. rupfen, to pluck, than to the present word.]