"A mutton-headed old mugwump" - such was Boris Johnson's description of Jeremy Corbyn in an interview on Good Morning Britain today.
The Foreign Secretary is well known for his love of obscure Victorianisms in everyday speech, but this was an unusual put-down even by his own standards.
His use of the word "mugwump" caused an immediate stir on social media, with Twitter users attempting to work out exactly what Johnson was trying to say about the Labour leader.
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According to the Merriam-Webster dictionary, a mugwump is "a person who is independent (as in politics) or who remains undecided or neutral".
This sense is probably the origin of Johnson's use of the word, says the Daily Telegraph. Mugwump "can be used to imply that someone is "holier-than-thou'", an accusation frequently levelled at Corbyn.
However, some politically-savvy tweeters claimed Johnson had shot himself in the foot.
Labour Party activist Christian DeFeo says the term's use in politics dates back to the 1800s, when it was applied to rebels who deserted the Republican Party over its corruption.
Mugwump is also thought to derive from the Native American word "mugquomp", used by the north-eastern Algonquian people to describe a war leader or chief, or a person of importance.
For the millennials who dominate Twitter, a "mugwump" can only mean one thing - the head of the International Confederation of Wizards in the Harry Potter books.
JK Rowling borrowed the term for her novels and as her Supreme Mugwump is the head of the wizarding equivalent of the UN, several Twitter users are amused by Johnson's apparent insinuation that Corbyn is a top magician.
Johnson embraced the online hullabaloo in a later appearance on BBC Radio 4's Today programme, offering a tongue-in-cheek apology "to mugwumps everywhere" for his use of the term.
The Foreign Secretary is well known for his creative and sometimes controversial turns of phrase.
He once likened Tory in-fighting to "Papua New Guinea-style orgies of cannibalism and chief-killing" and told a Tel Aviv audience that boycotts on Israel were perpetrated by "corduroy-jacketed, snaggle-toothed lefty academics".
When it comes to creative insults, though, Johnson's finest hour came last year, when he won £1,000 for penning the best offensive poem about censor-happy Turkish President Recep Tayyip Erdogan in a competition run by The Spectator magazine.
His winning limerick began: "There was a young fellow from Ankara, who was a terrific w***erer".
