The jocular British-English and Irish-English phrase to get one’s knickers in a twist means: to become unduly agitated or angry. The plural noun knickers (short for knickerbockers) denotes short underpants worn by women or girls.
—Cf. also, here, the variant to get one’s panties in a twist—and cf. the synonymous American-English phrase to get one’s panties in a bunch (the plural noun panties denotes short underpants worn by women or girls).

In the phrase to get one’s knickers in a twist, twisted clothing is a metaphor for mental confusion. The English novelist George Eliot (Mary Ann Evans – 1819-80) had already used this image in Romola (London, 1863)—Fra Francesco, a Franciscan preacher, has challenged the Dominican preacher Girolamo Savonarola (1452-98) to walk through the fire so as to prove the divine origin of his doctrines by coming out unhurt:

“It’s the Frate’s doctrines that he’s to prove by being burned,” said that large public character Goro […].
“Nay, Goro,” said a sleek shopkeeper, compassionately, “thou hast got thy legs into twisted hose there. The Frate has to prove his doctrines by not being burned: he is to walk through the fire, and come out on the other side sound and whole.”

The earliest occurrence that I have found of to get one’s knickers in a twist is from the comic strip Andy Capp, by the British cartoonist Reginald Smyth (1917-98), published in the Daily Mirror (London) of Tuesday 26^th January 1971—a rent collector is at Andy’s door:

– Andy (from inside the house): Simmer down, lad – Come inside an’ we’ll discuss what I owe yer over a brandy.
– Rent collector: Grr! Grrr! Yer can’t afford t’ pay yer debts but yer can afford t’ drink brandy!!
– Andy (appearing on the doorstep with two glasses of brandy): Don’t get yer knickers in a twist, mate – I ’aven’t paid for this, either.

The second-earliest instance of the phrase is from the Morning Star (London) of Saturday 26^th June 1971:

Britain’s Foreign Office mandarins have had their knickers in a twist for the past fortnight over publication of the secret Pentagon papers about the Vietnam war.

In the letter to her eleven-year-old daughter published in the Evening Chronicle (Newcastle upon Tyne, Northumberland) of Thursday 17^th May 1973, the novelist Lee Langley (born 1932) confirmed that the phrase gained currency in 1971:

Dear Sarah,
When you read this letter I can guess what your reaction will be: eyes cast up to heaven, shoulders lifted in a slow shrug, your verdict will emerge in one word: “Pathetic!”
Last year’s word was a heavily sarcastic “Charming!” (the stock response to anything from “your face needs washing” to “could you make your bed?”), which was at least an improvement on the previous year’s “Okay ducky, don’t get your knickers in a twist”.

Don’t Get Your Knickers in a Twist is the title of an X-rated film mentioned, for example, in the cinema programmes published in The Observer (London) of Sunday 11^th February 1973. John Slim alluded to that film in his column Postscript, in The Birmingham Post (Birmingham, Warwickshire) of Wednesday 6^th June 1973:

Choice words
There’s no business like show business. Peter Johnson, general manager of the Jacey Cinemas group, reports receiving a telephone call from an irate woman who had been frustrated in all her previous efforts to ascertain what was on at a particular cinema.
From the moment he answered the telephone, she began to enlarge explosively on her problems, allowing her one-man audience no chance to join in. Eventually, however, she asked: “And can you tell me what’s on . . . ?
Mr. Johnson replied: “Don’t Get Your Knickers in a Twist.”
It didn’t help.

The Daily Mirror (London) of Wednesday 7^th March 1973 published an advertisement for ‘Y-Front’ by Lyle & Scott, which punned on the phrase:

Don’t get his knickers in a twist.
Get him the real thing

The real thing is ‘Y-Front’! And ‘Y-Front’ have been in the business long enough to know what a man needs.
A real ‘Y-Front’ garment superbly achieves that proper blend of softness and absorbency. All ‘Y-Front’ fit precisely. And, therefore, comfortably. They look good,
They last and last.
But make sure it’s genuine ‘Y-Front’. Look for the name on the bag and the garment.
With ‘Y-Front’ there are no hang-ups. Or hang-downs. So next time you buy his underwear, don’t get it where you get your fish fingers. Or even where you get your own underwear.
Go to a man’s shop and ask for the real thing. Say ‘Y-Front’. Firmly. He’ll feel better for it.

Peter Mortimer elaborated on the phrase in a film review published in The Journal (Newcastle upon Tyne, Northumberland) of Friday 8^th June 1973:

“Trip To Kill” (Classic Two, Newcastle, Sunday), gets its moral knickers in a bit of a twist. It’s a fast moving razmataz affair about ex-cop Joe Ryan (Tom Stern), who returns from Vietnam after being decorated for his heroic deed in diving on top of a grenade — it didn’t explode.
Back in Los Angeles, and disenchanted, he becomes a long-haired drop-out, smoking soft dope. By a series of incidents, a mysterious narcotics investigator, Frank Redford (Telly Savalas) implicates Ryan in a plan to capture hard drugs king Henry Neilsen (Robert Vaughn).
The finale is a Hitchcock-type setting as Ryan is pursued to deserted amphitheatre to fight out the gory last act with the drugs men.
There’s no doubt the film moves at a good pace, and it’s imaginative directing from Tom Stern. But the film’s dubious morality seems based on quicksand.

__________

Added on Saturday 7^th February 2026:

I have found only a few occurrences of the variant to get one’s panties in a twist in Irish English and British English—these are the first two:

1-: From an article by Eve Wisdom, published in the East Antrim Times (Larne, County Antrim, Northern Ireland) of Friday 19^th November 1971 [page 6, column 1]:

Beauty lark—why get your panties in a twist about it?

I suppose there’s nothing wrong in changing your mind, especially if the subject isn’t too serious.
Now take this Miss World lark which was run last week. I can well remember sounding off about it and joining the side of the house who declared it was nothing short of a cattle mart.
Then last year I saw the “burn the bra for liberty brigade” throwing flour bombs on to the contest cat-walk during the show to demonstrate their militant views.
Then I thought to myself, Miss World or no Miss World, what’s the odds? Why get your panties in a twist about it? It’s all a bit of fun.

2-: From the column Help! A service for consumers, by Margaret Jones, published in the Daily Mirror (London, England) of Tuesday 23^rd July 1974 [page 24 [?], columns 6 & 7]:

PANTIE IN A TWIST
I bought a pantie girdle after trying it on in the shop. After a week I wore it and found it was not really suitable. The shop won’t change it as I’ve no receipt and no proof that I bought it there. What can I do?—Mrs. B. W., High Wycombe, Bucks.
● Even if you had a receipt, the shop wouldn’t be obliged to change it because you had a chance to see whether it was suitable at the time of buying.