Short on the sides, longer on top.  For decades, those seven words, understood in every barbershop in the land, were told to the person wielding the clippers. Not anymore. Now the style is called the disconnected undercut, and these days it’s everywhere.  Nick Wooster, the men’s fashion industry veteran, has a disconnected undercut. So does Nathan McCallum, the men’s style blogger.  The Justins Timberlake and Bieber, along with Brad Pitt, are credited with popularizing the look, though it isn’t limited to men. The most influential disconnected undercut right now might belong to Ruby Rose, the actress who stars on “Orange Is the New Black.”  Despite its technical-sounding name, the cut is basic. The sides are shaved with clippers set to one or zero. The top is left long. And unlike a classic blend or fade, the transition from short to long hair is abrupt — that is, disconnected.  Jordan O’Brien, a San Francisco-based YouTube style vlogger who has the haircut, said it’s a can’t-miss look. “The reason people have it is it’s so versatile,” Mr. O’Brien said. “And it looks good on anybody.”  Indeed, with the tuft on top you can do just about anything, from slicked back with gel; to a messy, beachy approach; to a classic side part that feels 1940s-era G.I., like Mr. Pitt’s look in the film “Fury.”  The major league pitcher A. J. Burnett wears a kind of bushy disconnected mohawk. The musician Sam Hunt shaved lines into his short sides. Some guys grow the top long and use styling product to swoop their hair into a disconnected pompadour, as did David Beckham in a recent H&M campaign. This was a new take on the disconnected undercut he has worn for years, arguably starting the trend.  The look’s godfather may be the director David Lynch, who has been short on the sides and longer on top for years.  Justin Lefkovitch, the chief executive of Mirrored Media, a music marketing firm based in Santa Monica, Calif., said that he went in a completely different direction, with a disconnected undercut that is “part of that man-bun family.”  “My hair is a little unruly, so I prefer not to do the greasy slicked-back look,” Mr. Lefkovitch said. “It’s a super relaxed, bunned up into a ball in the back. I call it the disconnected topknot.”  Still, Mr. Lefkovitch said the look isn’t so wild that it turns heads in business meetings. His hairstylist, Jamie Pierce, agreed.  Ms. Pierce, who works at the Doves presented by Studio DNA salon in Santa Monica, said that she has been asked to do so many disconnected undercuts in the last year that she has become an expert at it. The style allows conservative older guys to be a little fashion-forward, she said, describing it as “barbering with a twist.”  Still, maintaining the haircut isn’t as simple as the ear buzzing you received at the barbershop as a kid. Kyle Schaffler, a fashion publicist in New York, said he had to learn how to use a round-head bristle brush in conjunction with a blow dryer to shape the top tuft so it wouldn’t flop around.  “You have to do something with it to give it some style,” Mr. Schaffler said, adding that during the workweek he blow-dries it back for a professional look.  Asked if he thought the disconnected undercut was as universal as touted, Mr. Schaffler said it depended on the guy.  “Nick Wooster rocking this haircut, that’s so cool,” he said. “If my dad was rocking this hairstyle, I’d say: ‘What are you doing? We’re going to the barber right now.’ ”
You’ve seen it around: tight on the sides, free-flowing on top. From Justin Bieber to Nick Wooster, the haircut has captured the imaginations of the masses.