A coworker, unprompted, over lunch: "Lately my boom is oat milk lattes."
Nobody asks how long the boom has been running. Nobody asks if she's "really into it" or just trying it. The sentence already answered both questions.
Three weeks later she's back to regular coffee and nobody mentions that either. The boom simply ended, the way booms do, and saying so out loud was never going to be awkward.
"Boom" arrived in Japanese doing its normal English job — an economic boom, a boom in demand — a big, public, society-sized surge. Then in 1994 the comic essayist and self-described "otaku king" Miura Jun went on a Fuji TV talk show and used it to describe something much smaller: not the nation's current obsession, but his own. My boom. The idea that a single person could have a private version of a national fad, and that it deserved the same word, was funny enough and useful enough that it won a spot on Japan's annual new-slang awards list in 1997 and never really left.
The genius is in what "my" is doing to "boom". A boom, by definition, is supposed to be collective — a trend, a bubble, something the whole country is briefly doing at once. Bolting "my" onto the front of it is a small, deliberate joke: I am declaring a market trend, population one. It borrows the scale and energy of a national craze and applies it to something as small as a snack you've been buying every day this week.
What makes it genuinely useful, not just cute, is the built-in permission to stop. English hands you "hobby", which implies commitment, or "phase", which a parent uses to dismiss something before it's even finished. Mai buumu carries no such judgment in either direction. Declaring one is not a promise and abandoning one needs no explanation, because everyone already agreed, the moment the word was used, that the feeling was temporary and that this was fine.
Which is why it gets used for absolutely everything, at every age, completely without irony. A teenager's mai buumu might be a K-pop group. A retired grandmother's might be growing shiso on her balcony, or a specific brand of barley tea, or, memorably, according to more than one Japanese magazine profile, a particular philosopher she'd never read before turning seventy. The size of the obsession and the seriousness of the person carrying it are both irrelevant. Only the temperature and the honesty about its shelf life matter.
English has plenty of words for a fleeting interest, and almost all of them are faintly embarrassed about it — "phase", "fad", "kick" — words other people use to describe your enthusiasm back at you, usually while implying you'll grow out of it. Mai buumu is the same idea with the shame surgically removed, and the fact that Japanese needed to build it, rather than just importing "my hobby" or "my thing", says something about how much cultural work goes into that removal. It's a word that lets you commit fully to something for exactly as long as it lasts, in public, without anyone — including you — treating the ending as a failure.