The supremely confident billionaire hedge-fund manager Bill Ackman has never been afraid to bet the farm that he’s right.

In 1984, when he was a junior at Horace Greeley High School, in affluent Chappaqua, New York, he wagered his father $2,000 that he would score a perfect 800 on the verbal section of the S.A.T. The gamble was everything Ackman had saved up from his Bar Mitzvah gift money and his allowance for doing household chores. “I was a little bit of a cocky kid,” he admits, with uncharacteristic understatement.

Tall, athletic, handsome with cerulean eyes, he was the kind of hyper-ambitious kid other kids loved to hate and just the type to make a big wager with no margin for error. But on the night before the S.A.T., his father took pity on him and canceled the bet. “I would’ve lost it,” Ackman concedes. He got a 780 on the verbal and a 750 on the math. “One wrong on the verbal, three wrong on the math,” he muses. “I’m still convinced some of the questions were wrong.”

Not much has changed in the nearly 28 years since Ackman graduated from high school, except that his hair has gone prematurely silver. He still has an uncanny knack for making bold, brash pronouncements and for pissing people off. Nowhere is that more apparent than in the current, hugely public fight he and his $12 billion hedge fund, Pershing Square Capital, are waging over the Los Angeles–based company Herbalife Ltd., which sells weight-loss products and nutritional supplements using a network of independent distributors. Like Amway, Tupperware, and Avon, Herbalife is known as “a multi-level marketer,” or MLM, with no retail stores. Instead, it ships its products to outlets in 88 countries, and the centers recruit salespeople, who buy the product and then try to resell it for a profit to friends and acquaintances.

Ackman has called Herbalife a “fraud,” “a pyramid scheme,” and a “modern-day version of a Ponzi scheme” that should be put out of business by federal regulators. He says he is certain Herbalife’s stock, which in mid-February traded around $40 per share, will go to zero, and he has bet more than a billion dollars of his own and his investors’ money on just that outcome. (Ackman declines to discuss the specifics of his trade.) “This is the highest conviction I’ve ever had about any investment I’ve ever made,” he announced on Bloomberg TV. An interviewer on CNN reminded him that Herbalife had been around since 1980 and had withstood many previous challenges to its business plan. How long was he willing to “sit on this bet”? Ackman replied, “We’re not sitting. We’re shouting from the rooftops. They’ve never had someone like me prepared to say the truth about the company. I’m going to the end of the earth. If the government comes out and determines this is a completely legal business, then I will lobby Congress for them to change the law. I had a moral obligation. If you knew that Bernie Madoff was running a Ponzi scheme, and you didn’t tell anyone about it, and it went on for 33 years ... ”

Ackman says he suspected, when he “shorted” (i.e., bet against) Herbalife, that other hedge-fund investors would likely see the move as an opportunity to make money by taking the other side of his bet. What he hadn’t counted on, though, was that there would be a personal tinge to it. It was as if his colleagues had finally found a way to express publicly how irritating they have found Ackman all these years. Here finally was a chance to get back at him and make some money at the same time. The perfect trade. These days the Schadenfreude in the rarefied hedge-fund world in Midtown Manhattan is so thick it’s intoxicating.