Matthew McConaughey has been singing the praises of bumbags (shame he doesn't know how to wear one)

Harriet Walker explains the dos and don'ts of fanny packs

Shooting from the hip: the ‘True Detective’ star was quick to defend his use of the bumbag (Getty Images)

The bumbag (see also: fanny pack; chaos pouch) comes and goes with all the vigour of a fashionable bout of flu – you can see it happening around you and want no part of it, but inevitably end up just another nose among the sneezing masses.

Have some empathy, then, for poor Matthew McConaughey, who seemed to be on such a roll with good choices recently that he's no doubt due a shocker. He's the latest celebrity to have succumbed to a hands-free utility device.

In the words of Jerry Seinfeld: it looks like your belt is digesting a small animal.

"How many times are you around someone and they're like, 'I forgot my so and so?'" he told reporters, whose keen eyes for a story homed in on the black leather pouch almost immediately. "I'm like, 'I got mine right here.'"

What McConaughey has failed to realise is that you get this sort of effect with any style of bag, designed as they are to carry one's personal effects.

"You gotta kind of put it on the side to make it look a little not as nerdy, but still, practicality wins out," he added. Note he is careful to pose his bumbag as "a little not as nerdy" rather than "not nerdy". Not even his True Detective character, Rust Cohle, could argue that case.

And yet, unlike that same character, McConaughey isn't emphatically, brutally and almost existentially alone in his quest, for all that we, the trend-guzzling public, might wish him to be. Other recent fanny-packers include singer Rihanna, model Cara Delevingne and popstrel Rita Ora, a trio of streetwise scenesters whose fashion choices are co-opted the world over. By people young enough not to remember the last influx of bumbags, that is.

"I'm into them," says Kay Barron, the fashion-features director of Porter magazine. "The thing is the name. If you say you're wearing a bumbag, it will always sound like an ironic fashion accessory, but if you say you are wearing a beltbag, it's suddenly totally acceptable.'

Purple reign: Singer Rihanna models a Chanel bumbag (Rex Features)

There's something undeniably sassy about them. They're for people at their most efficient, whether they're dancing to Kris Kross or simply reading a map in the middle of a busy street in everybody else's way. And there are a couple of concepts at work in confluence here that go some way to explaining the phenomenon.

It's festival season, sure, a time of year when anyone can trip up and land headfirst in fashion slurry. Glastonbury has a bumbag count that is higher per capita than even that of Iowa's biggest nerd convention. You can see the appeal – they leave both hands free to drink a beer, light a cigarette and waft away the cloud of flies emanating from the midden just to the left of the main stage.

Then there's "normcore", too, one of the least inspiring youth movements to have gained traction since the Young Conservatives, which dictates that in order to function outside the trendy corporate machine, one must dress like an American tourist – beneath the sagging girth of which you'll most usually find a bumbag.

But there's a fashion element to it, too – Fendi and Chanel have fielded bumbags in the past. Carrie Bradshaw wore one once. Dolce & Gabbana styled theirs with flowing, floor-length silken evening gowns. The rule seems to be, if you're going to go for the high-end look, yours must be as impractical as possible.

So, in fact, Matthew McConaughey's stylised hip-spin is about as far from a good idea as it's possible to be. Unless, of course, you are a window cleaner.