For most viewers of Happy Days, the wildly popular ABC sitcom of the 1970s and early 1980s, the sight of Arthur “Fonzie” Fonzarelli jumping over a shark on water skis during the September 20, 1977 episode was not a momentous event. It was simply agreeably silly—a result of the Fonz taking up the challenge of a local beach bully to endanger his life with an ocean predator. Yet the Fonz’s machismo would come to define a moment in pop culture when a once-beloved creation takes a noticeable dip in quality.

Premiering in January 1974, Happy Days was a perfect storm of sitcom affability, from Ron Howard’s Richie Cunningham to Donny Most’s Ralph Malph to Anson Williams' Potsie Weber. Created by Garry Marshall, the series was a homage to the relative innocence of 1950s middle America, recreated for 1970s viewers soured by the Vietnam War and entering decidedly less innocent times. By the time the show entered its fifth season in the fall of 1977, it was firmly entrenched as a showcase for Henry Winkler’s Fonzie, who was originally set to be a supporting character. Winkler was such a hit with viewers that the edges of his menacing biker were smoothed out, and Fonzie became a lunch box icon.

By the time the three-part “Hollywood” season premiere aired, part of a ratings stunt that had the cast visiting California, Fonzie could do no wrong—up to and including a leap over a tiger shark in response to a dare from a local beach bum named the California Kid. The idea for the scene actually originated with Winkler, who was an avid water skier. According to Winkler, his father insisted his son tell the producers he could water ski. When Winkler finally relented, they wrote him a scene in which he lands a jump over a shark contained in a netted area.

“I did all the waterskiing—except for the jump,” Winkler told Oprah Winfrey in 2015. “They wouldn’t allow me to ... Well, I also couldn’t do the jump! I don’t know how to do that.”

By this point, Happy Days was a smash, and 30 million viewers watched as the Fonz was crowned king of the beach. While some may believe it was the beginning of the end for the series, the show was only midway through its run. It aired for another six seasons.

It wasn’t until 1987, when University of Michigan college student Sean Connolly coined the phrase “jumping the shark” to describe a particularly outlandish turn of events that Fonzie’s beach exploits started to take on a new meaning. Their circle of friends used the expression for years. In 1997, Connolly’s college roommate Jon Hein started the website JumptheShark.com, which chronicled the moments when beloved television shows took a sudden and alarming dip in quality.

Fred Fox Jr., who wrote the episode, chafed at the idea that it signaled a downturn for the show. In 2010, he wrote an op-ed for the Los Angeles Times objecting to the phrase. “That’s why, when I first heard the phrase and found out what it meant, I was incredulous,” Fox wrote. “Then my incredulity turned into amazement. I started thinking about the thousands of television shows that had been on the air since the medium began. And out of all of those, the Happy Days episode in which Fonzie jumps over a shark is the one to be singled out? This made no sense.”

Fox may have a point. Fonzie’s aquatic adventures weren’t even the most bizarre element of Happy Days that season. Later on, the show introduced Robin Williams as Mork, an alien from the planet Ork. The character was a hit, and Williams went on to star in Mork and Mindy. One would think the arrival of an alien in an otherwise grounded sitcom about teenagers would be the real shark-jumping moment, but “meeting Mork” doesn’t have quite the same ring to it.