It was in the crowd when Donald Trump met the Irish prime minister on St Patrick’s Day. It was also on the pitch when Manchester City played Liverpool 10 days ago, thanks to Argentinian defender Nicolás Otamendi, while Aston Villa’s Neil Taylor also sports it. Dior model Cole Mohr has it, as does Richard Spencer, white nationalist and self-nominated face of the “alt-right”, and so, it could be argued, does Labour MP Keir Starmer. It’s the buzziest buzzcut in men’s hair.

As yet, this look, with its hard parting, is known only as that: “the hard part”. But, according to Irish barber Craig Nolan, a founding father of the look who has been championing it for two years at his Dublin salon, it is now close to peaking. Nolan is at least partly to thank for Irish UFC fighter Conor McGregor’s recent transformation into a style icon – as his barber, he has given him a hard part.

The cut has two main components: a skin fade (a close shave which blends hair to the skin) around the sides and a very distinct sharp, angular parting, finished with a smear of Brylcreem on top. Nolan says it takes less than 10 minutes to do, and requires only a razor and scissors. And, in the case of City’s Otamendi, sometimes an extra line shaved in on the sweet spot, just above the temple.

The hard part in detail: Nicolás Otamendi’s haircut explained. Photograph: Getty Images/Guardian Design Team

Undercuts are one of the most lucid markers of the much-maligned hipster, being a haircut that references the past. But while other “retro” signifiers – plaid shirts, beards and naval tattoos – have moved towards parody, the hard part marks the next phase of undercut; being both heritage and modern, it moves the look forward. See Beckham’s disconnected undercut, long on top and short on the side. “It combines both old-school styling with modern techniques, creating a very clean, sharp and timeless look,” explains Leah Hayden Cassidy, an Irish barber who also works at the Nomad salon in Berlin, where the cut is also popular. That it has taken off in Irish barbering is odd and is most likely an Instagram thing: “There’s a community, I suppose, but it’s one of the first times a haircut has been bigger in the UK and Ireland than in the States,” offers Nolan.

The cut has cropped up in various incarnations over the years. It was a common look in the US in the 1930s, for example, and its popularity among bands such as New Order and Duran Duran earned it the name “the synth” in Sweden. Hayden Cassidy references the 1950s with hers. Paddy Corrigan, of Jack’s Barbers in Dublin, says he also draws inspiration from 1990s youth.

But if these cultural references are familiar, there is one that is far less comfortable – the similarities with the haircut seen in photos of Hitler Youth. One barber in New York cited customers asking for an early variation of this very cut to save time. Spencer aside, it is unlikely that any of these wearers are alluding to fascism. Nolan, for one, thinks it is more about the weather: “There’s a lot of shaving, so it tends to pick up as it gets warmer,” he says.