The unhappy wife of a well-known, workaholic Wall Street trader, a man who is out with clients practically every night, passed by Tiffany’s and saw a $10,000 necklace she liked. So, ignoring an agreement with her husband that she would never make a big-bucks purchase without first checking with him, she went into the store and bought it. This incident can be classified as financial infidelity, a phenomenon that is a ballooning part of the American economy, especially in New York City. I heard about it from Bonnie Eaker Weil, a prominent city therapist for 30 years who specializes in couples’ and relationship therapy at a cost of $250 an hour.

Financial infidelity is defined as a hidden, undisclosed big-time expenditure by someone who is either married or lives with someone and suddenly makes what Dr. Weil describes as “a POP, or a pissed-off purchase,” because their partner has lied to them, cheated on them, or has gotten in a fight with them.

Dr. Weil, who performs telephone therapy with clients in such countries as Israel, Australia, Italy, and England, also told me about a financial infidelity incident concerning another client, an attorney at a top New York law firm. The lawyer went golfing with some buddies in the Hamptons, and while there saw what he described to Dr. Weil as “a drop dead $2 million home in Southampton that he couldn’t resist.” So he put down a binding $50,000 deposit. In doing so, he, too, broke a standing agreement — that he and his wife would discuss any major purchase with the other prior to making it. His wife, Dr. Weil says, was incensed, and she, in turn, went to one of the city’s top plastic surgeons and paid $100,000 up front for a “top to bottom overhaul” without informing her husband.

What makes these incidents so intriguing is that financial infidelity — unfamiliar to most people and estimated to run an astonishing $424 billion a year — is “a trend that’s sweeping America,” Dr. Weil tells me. She claims to document the trend in a just-completed two-week study centered on a survey of 400 couples across the country in all income brackets.

Dr. Weil, who conducted her survey in conjunction with Castek International, a marketing and research firm operating out of Mt. Vernon, N.Y., undertook the study to time it with the publication of a book she has written, aptly called “Financial Infidelity,” which will hit book stores on Thursday.

Spending arising from financial infidelity “is growing like crazy, and like spending on food and medicine, it should transcend any economic downturn,” Dr. Weil says.

Utilizing the findings from the survey as a means of establishing a legitimate national expenditure figure on financial infidelity, Castek’s CEO, Andre Wallace, places the amount at about $424 billion. This figure was arrived at by applying the results to the total American household population of nearly 60 million couples — 54.4 million married households and 5.4 million unmarried-partner households. Of this population, about 594,000 are same-sex couples.

The study determined that the average couple nationally produces four POPs a year at about $486 a POP. In Manhattan, the number of yearly POPs a couple averages eight and runs nearly $1,000 each. In her book, a breezy, lively, and informative read, financial infidelity is described by Dr. Weil as the no.1 relationship wrecker, one that can destroy love, confidence, and trust. She offers a variety of solutions to battle this threat.

Meanwhile, it grows bigger and bigger with no letup in sight.

dandordan@aol.com