Not the jar, the idea
It is hard to think of another food brand that became a word in normal British English. Hoover, in its day. Tippex. That is most of the list. Marmite is in it.
If you describe a film, a politician, a footballer or a piece of new architecture as “a bit Marmite”, everyone in the room nods. They know what you mean and they do not need a follow-up question. That is unusual for something whose principal job is being spread thinly on toast.
Where it came from
The phrase dates from the 1996 “Love it or hate it” campaign, which was less an invention than a recognition. People already felt strongly about the spread. The clever bit was leaning into that instead of pretending the jar had broad appeal. Bartle Bogle Hegarty took over the account a few years later and held the line.
What the campaign did, almost as a side effect, was give people a noun for a particular shape of split opinion. A Marmite book is one half the book club worships and the other half cannot finish. A Marmite manager is the kind your team will follow off a cliff or threaten to resign over. A Marmite restaurant gets either five stars or one. No threes.
A small literary aside
There is a slim volume by Maggie Hall called The Mish-mash Dictionary of Marmite: An Anecdotal A-Z of “Tar-in-a-Jar” that collects Marmite trivia. It is a love letter, not a reference work, and it is a useful present for the right kind of person. You will know which kind because they will not stop talking about Marmite.
The jar also turns up as stage business in British television. A breakfast scene with a jar of Marmite on the table is doing two things at once: it fixes the scene in Britain, and it is telling you something quiet about the character.
The slogan walks out the door
The funny thing about “Love it or hate it” is that the agency probably did not set out to write a tagline that would escape and become a public idiom. Brands try to make that happen all the time, with bigger budgets and worse outcomes. Marmite did it almost by accident, which is more or less the only way these things ever really happen.
The metaphor will outlast some of the things currently being described with it. That tends to happen when a product is interesting enough to disagree about.
