Nickname for a kind of fraud or confidence game. More specifically, a kind of bill fraud.
An insider on the grift, usually employed in accounts receivable or as a cashier, embezzles a cash payment from one customer and then covers that customer's payment with the next cash payment that comes in. The second customer's payment is subsequently covered by the next customer's payment and so on.
These overlapping payments create a float of money that can go undetected as long as all customer payments are eventually posted. In many companies and firms, a post date from two days to several weeks after the payment is normal, so a fairly large float can be accumulated in this way. Alternatively, in a busy enough company, several payments can be lapped until enough time has passed that the relevant ledgers are in archive. Once an auditor is called in, it is probably time for the insider to skip town.
Most of what makes this possible is the lack of oversight and rigor in the accounting department, which arises from the conflict between keeping costs low and due diligence.
This process likely served as inspiration for the infamous Ponzi Scheme, which often involves paying the return on one investor's principal with another investor's investment.
One etymology of the term has it coming from "overlaping" (i.e. overlapping payments). Another etymology asserts that the small payments removed from the till are like an animal's tiny laps from a bowl of water or milk. Since grifter slang is largely anecdotal and often popularized by the works of fiction writers, precise etymology is difficult to determine. This author leans toward the former explanation.