YOLO had become the most frequently mentioned slang term among the students, just edging out “totes” for “totally” and “cray” (or “cray-cray”) for “crazy.”

What accounts for the meteoric rise of YOLO, and how has it gone virtually unnoticed by nonmillennials? Its appeal to the youthful is self-evident. YOLO as a shorthand mantra defines youth, on a certain level. What is teenagehood if not the adventurous, often foolhardy, desire to test the limits of acceptable behavior—because hey, why not? YOLO!

The carpe diem sentiment of “you only live once” has a long history predating its punchy acronymic clipping. Garson O’Toole, on his Quote Investigator blog, traces variations on the theme back to the 18th century, as in this line from Samuel Richardson’s novel “Clarissa”: “We live but once in this world; and when gone, are gone from it for ever.” The exact wording of “you only live once” begins cropping up in the late 19th century, and by 1937 it was popular enough to be used as the title of a Fritz Lang film noir. One wry elaboration, credited to the comedian Joe E. Lewis in 1952, is “You only live once, but if you work it right, once is enough.”

Today’s young purveyors of YOLO do indeed appear to be trying to work it right, though what that means is up for debate. Eble’s students illustrate its typical use, as a carefree tag to explain an impulsive choice: “You want to park illegally in this spot? YOLO!” “Should I buy these shoes or pay rent? YOLO!” The word has also found favor as a verb, as in this Yelp review of a Jersey City pizzeria: “All the times I come home after a night of YOLO-ing, I crave a delish slice to soak up the booze and sober me up.”

But as the term has circulated over the past several months, a YOLO backlash has set in. Jason Salcedo, a high school senior from Stuart, Fla., recently blogged that YOLO is now “used by teens only as an absolute justification to do dangerous or harmful things.” As he pointed out, those who use it might not appreciate that you only die once, too.

Even as teenagers argue over the merits of YOLO-ing, their parents and teachers remain largely oblivious. Slang serves a powerful function for marking an “in-group,” as sociologists say, and it’s easy to see how YOLO would be precisely the type of term that its users would want to keep to themselves, away from elders.

One elder who has taken notice is David McCullough Jr., an English teacher at Wellesley High School (and son of the Pulitzer Prize-winning historian). “Now, before you dash off and get your YOLO tattoo,” he said in his caustic commencement address for the school in June, “let me point out the illogic of that trendy little expression—because you can and should live not merely once, but every day of your life. Rather than You Only Live Once, it should be You Live Only Once...but because YLOO doesn’t have the same ring, we shrug and decide it doesn’t matter.”