The story
rate + tart i.e., rate, as in "rate of interest", and tart, UK slang for a prostitute, referring to the promiscuity (in the sense of indiscriminate choice) of such a person.
Etymology from Wiktionary, CC BY-SA 4.0.
rate tart
Today it means: A holder of a credit card or bank account who frequently and intentionally changes providers to take advantage of an offer of a low interest rate established to attract new customers. Full definition →