bet dieting
pp. Betting money on losing weight, particularly where the money goes to a charity or other organization that one disagrees with.
Other Forms
Most of the seasonal customs Drake-Carnell records appear, like my Bank Holidays, to involve cakes and ale in some combination. Some of them in bizarre conjunction with penny-throwing or ancient religious, possibly even pagan, observance. But none of them combine food, money and cultish devotion like the contemporary craze that is bet dieting.

It was reported this weekend that slimmers are signing up for a website called StickK, on which they post their target weight and pledge to donate money to a cause they hate if they fail to meet their target. Apparently this provides dieters with the extra incentive necessary to shift the last stubborn pounds.
—Michael Gove, “Ye olde custom: cursing of the SatNav,” The Times (London), May 25, 2009
Bet dieting is the newest rage and there are a few websites
that enable it, but stickK has an extra ploy: the "anti-charity." Choosing the most politically controversial non-profit charities to motivate someone to achieve their goals is a great idea. Science and the stock market know that risk is a much more powerful motivator than reward.
—Boomer Babe, “Bet Dieting: If You Lose You Pay Guns, God, & Gays!,” InventorSpot, May 23, 2009
2009 (earliest)
The founders say a particularly effective way to encourage people to lose weight is to get them to nominate a charity with whose views they disagree to receive money should they fail.

Jordan Goldberg, co-founder of the StickK bet dieting website, said: "The anti-charity aspect is where we take your money and we send it to an organisation that you oppose should you fail.

"We chose some highly contentious issues, for instance global warming, abortion and gay marriage.
—“Bet dieting takes hold in the UK,” BBC News, May 23, 2009
With a month to go, we weighed in. Me: 217 pounds, down 15 1/2 pounds. Finley had lost 14 3/4 pounds. Pressman and Carroll were both short of the 10-pound loss mark.

Carroll was playing a deep game, planning to start slow and finish fast, thereby lulling his opponents into a false sense of security. He had not told his herb doctor about the diet bet because he thought his herb doctor would consider it frivolous to play at dice with the nutritional universe.
—Michael Robertson, “Dieting Under Duress,” The San Francisco Chronicle, March 28, 1986