"Lean in," thanks to the title of a new book by Facebook COO Sheryl Sandberg, has become "the idiom of the moment," Motoko Rich writes in the New York Times, adding "the phrase seems to have taken on a life of its own." But where did all of this "leaning in" come from?

In Sandberg's book, she uses a metaphor that she has been developing over the last few years on the theme of women pursuing their business ambitions in an assertive manner. Women in business are too often content to "lean back" when they should be "leaning in." Or as she told Barnard College graduates in her 2011 commencement address, "if all young women start to lean in, we can close the ambition gap right here, right now, if every single one of you leans in." She recommended that the young women "lean way into your career."

As I told the Times, the physical metaphor of "leaning back" and "leaning in" can be traced to any number of athletic activities. Searching on the phrase "lean in" or "lean into" will turn up many examples in aquatic sports, like surfing and sea kayaking, where it is often advisable to "lean into the wave." In winter sports like snowboarding and skiing, you might want to "lean into the turn." And "leaning in" is also seen as a good move in sports as diverse as rowing, boxing, football, baseball, horse-riding, riflery, motorcycling, and running.

From these different activities comes a general picture of "leaning in" (or "leaning forward") as a stance that suggests embracing risk and not shying away from difficulty or obstacles in one's path. It's not surprising, then, that "leaning in" should get applied to the business world. The earliest glimmer that I've found of this metaphorical transfer comes in a 1941 issue of Princeton Alumni Weekly. In the class notes, we learn that "Kent Cooper is leaning into it at Columbia business." By "leaning into it," I assume it means that young Mr. Cooper is giving it his all, applying himself, going for the gusto.

Even before Sandberg's book made a big splash, the "lean in" metaphor had found a home in business management circles. An Australian entrepreneur named Daniel Priestley often speaks of "leaning in" as the key to success; as he said in a post last year, "Lean in. All energy goes into moving forward." His book Entrepreneur Revolution, while not as high-profile as Sandberg's Lean In, also devotes space to the expression, explaining it in the context of snowboarding. But in an earlier interview with Priestley, the metaphor is said to be based on "leaning into the wave" in surfing. Regardless of the metaphorical basis of the phrase, "lean in" works as an exhortation to be confident, not unlike the overused phrase of last year, "double down."

In an online discussion of "leaning in," Dave Wilton, who runs Wordorigins.org, suggested a source independent of athletics. "A possible progenitor for the Sandberg's use of the phrase is from interactive television, which has for at least 15 years been describing the difference between 'lean back' and 'lean forward' to denote the difference between TV and the Internet," Wilton said. (That also helps to explain why MSNBC chose the slogan "Lean Forward," separating themselves from typical "lean-back" TV programming.) So it's possible that Sandberg picked up on this usage rather than one of the various sporting contexts. Either way, "lean in" is making waves.