If you want to make an impression at a board meeting or a Congressional hearing these bearish days, make a harrumphing noise and employ the figure of speech now sweeping the economic world: “But what about the fat tail?”

This is another way of asking “How come all you geniuses didn’t realize the risk you were running?” Embarrassed witnesses and recently fired C.E.O.’s explain that the distribution of values and risks long beloved by managers, credit-rating agencies and securities analysts turned out to be not so normal after all.

To comprehend what fat tail is in today’s media wringer, think of a bell curve, the line on a statistician’s chart that reflects “normal distribution.” It is tall and wide in the middle — where most people and things being studied almost always tend to be — and drops and flattens out at the bottom, where fewer are, making a shape on a graph resembling a bell. The extremities at the bottom left and right are called the tails; when they balloon instead of nearly vanishing as expected, the tails have been designated “heavy” and, more recently, the more pejorative “fat.” To a credit-agency statistician now living in a world of chagrin, the alliterative definition of a fat tail is “an abnormal agglomeration of angst.”

In an eye-popping Times Magazine article last month titled “Risk Mismanagement,” Joe Nocera, a business columnist for The Times, focused on the passionate, prescient warnings of the former options trader Nassim Nicholas Taleb, author of “The Black Swan” and “Fooled by Randomness,” who popularized the phrase now in vogue in its financial-­statistics sense. Nocera wrote: “What will cause you to lose billions instead of millions? Something rare, something you’ve never considered a possibility. Taleb calls these events ‘fat tails’ or ‘black swans,’ and he is convinced that they take place far more frequently than most human beings are willing to contemplate.”