This phrase is part of the current legal test for obscenity: material is said to be obscene if it "appeals to a prurient interest." A prurient interest is defined as "an unacceptable interest in sex." Unacceptable to whom?, one might ask. The answer is: to the judge or jury in a particular case. Oh well, the word comes from the Latin purire, meaning "to itch," and thus has the sense of an itching or burning desire.
Ever since the sub-prime debacle, the Prurient Interest rate has gone up to almost 5 percent.