The British-English noun brown Windsor soup (also shortened to brown Windsor) designates a thick brown meat-based soup of a type now often depreciatively depicted as emblematic of traditional British cookery.

The following, for example, is from the column In Passing, by the British author Geoffrey Bocca (1924-1983), published in The Daily Telegraph Magazine (London, England) of Friday 24^th October 1969 [page 67, column 3]:

The English have raised the art of bad cuisine to the point of literary expression. I can think of no other nation that could give its dishes names like Bubble and Squeak, Toad in the Hole, Soused Herring, Plum Duff, or take the sausage, that delicacy of infinite variety, and give it the generic name of “banger”.
[…]
[…] The following four-course English meal […] is almost a classic in unrelieved badness: Brown Windsor soup followed by pilchards in tomato sauce, shepherd’s pie and hot date (or fig) roly poly.

I will not go into the origin of the noun brown Windsor soup, because much has already been written about it—cf. in particular this interesting article: The muddy history of Brown Windsor Soup.

The earliest occurrences of the noun brown Windsor (soup) that I have found are as follows, in chronological order:
—Two preliminary notes: 1) None of the texts containing the earliest occurrences of the noun specified what brown Windsor soup consisted of; it is therefore impossible to know whether there was a single recipe or, on the contrary, various recipes. 2) The fact that cafés and restaurants advertised brown Windsor soup indicates that it must have been considered a tasty dish:

1-: From menus advertised by cafés and restaurants from 1919 to 1931:

1.1-: From the following advertisement for the Zetland Café, Stafford, published in The Staffordshire Advertiser (Stafford, Staffordshire, England) of Saturday 1^st November 1919 [page 8, column 3]:

MENU.
Table d’hôte Dinner 4/6.
From 5-30 to 7-30. Saturdays till 8-30.
Consommé or Brown Windsor Soup.
Fried Sole and Egg Sauce.
Jugged Hare, or Roast Sirloin of Beef. Yorkshire Pudding.
[&c.]

1.2-: From the following advertisement for Cadena Cafés Ltd., Southsea, published in The Evening News and Southern Daily Mail (Portsmouth, Hampshire, England) of Wednesday 24^th February 1926 [page 3, column 1]:

Special Table d’Hote Luncheon,
THURSDAY, FEB. 26^th, 1926.
2/6 LUNCHEON.
SOUP.
Tomato or Brown Windsor.
FISH.
Creme of Hake. Shrimp Sauce.
[&c.]

1.3 & 1.4-: From the following advertisements for Binns’ department store, Hartlepool, published in the Northern Daily Mail (Hartlepool, County Durham, England):

1.3-: Of Friday 21^st May 1926 [page 3, column 1]:

BINNS’ FAMOUS 4-COURSE LUNCH, 1/9.
To-morrow’s Menu:
Soup
Brown Windsor
Cream of Tomato
Entree
Steak and Kidney Pie
Braised Steak en Casserole.
[&c.]

1.4-: Of Wednesday 1^st February 1928 [page 1, column 2]:

To-morrow
SPECIAL LUNCH MENU
4 Courses, 1/9
SOUPS:
Green Pea Soup.
Brown Windsor.
ENTREE:
Steak & Kidney Pudding.
Grilled Pork Chops and Apple Sauce.
[&c.]

1.5 to 1.9-: From five similar advertisements for Binns’ department store, Hartlepool, published in the Northern Daily Mail (Hartlepool, County Durham, England) of Friday 16^th March 1928, Friday 27^th July 1928, Friday 3^rd August 1928, Friday 10^th August 1928 and Friday 21^st February 1930.

1.10-: From the following advertisement for Bobby’s, Bristol, published in The Western Daily Press (Bristol, England) of Friday 13^th February 1931 [page 9, column 1]:

To-day’s Menu
AT
Bobby’s
of QUEEN’S RD.
2/- LUNCHEON 2/-
Four Courses.
Soup.
Potage Brown Windsor.
Fish.
Fried Fillets Sole. Dutch Sauce.
[&c.]

2-: From other types of texts:

2.1-: From the diary of the U.S. socialite Wallis Simpson (born Wallis Warfield – 1896-1986), as published in Wallis Simpson’s Diary (1934) (Pen Pess, 2006), edited by Helen Batting [page 9]—The Little Man (TLM) was Edward, Prince of Wales; Cain was Wallis Simpson’s personal maid:

[Monday 1^st January 1934]:
Staff: chauffeur + motor gone already—but that needn’t be so bad if it makes The Little Man (TLM from now on) more likely to come up with a lift—and the housemaid going on Saturday. That’ll bring me down to Cain and the deaf (and dumb) cook, though I still haven’t given up on getting that treasure Mrs. Ralph back again from Lady Curzon. Let’s pray to God I can, or we’ll be on Brown Windsor for ever and a day.

2.2-: From The Art of Plain Living, published in Punch or The London Charivari (London, England) of Wednesday 30^th June 1937 [Vol. 192, page 706, column 2]:

Thick soups are constructed on a foundation of purée of potatoes. Vegetable soups are the most acceptable to the gourmet. These are generally white in colour and differ only in name, the flavour of all being identical. A few drops of cochineal will produce Crème de Tomates (Tomatoes). The Brown Windsor Soup so often found on British menus does not really exist at all; it is a misprint for Brown Windsor Soap [cf. footnote].

2.3-: From The Fancy (London: Michael Joseph, 1943), by the English author Monica Dickens (1915-1992) [chapter 13, page 222]:

After a time, he got tired of feeling nautical and went back along the pier to look for tea. All the teashops seemed to be shut or full. After queuing for a quarter of an hour for a seat, he shared a table with a woman whose idea of a suitable four o’clock meal was brown Windsor soup followed by prunes and custard.

2.4-: From the following advertisement for Newforge Limited, Belfast, published in the Lincolnshire Echo (Lincoln, Lincolnshire, England) of Monday 4^th October 1943 [page 2, column 6]:

WHAT MAKES A GOOD SOUP GOOD?
the answer is the Stock
that’s why
NEWFORGE CONCENTRATED SOUPS
are made from genuine rich concentrated meat stock
Ulster Broth ● Celery Soup ● Mulligatawny ● Oxtail ● Concentrated Meat Soup ● Pea Soup ● Brown Windsor ● 6½ d. and 8½. per tin.

2.5-: From The Western Daily Press and Bristol Mirror (Bristol, England) of Saturday 1^st January 1944 [page 4, column 3]:

ACKNOWLEDGMENTS
THE ROYAL INFIRMARY CHRISTMAS FUND

The Matron gratefully acknowledges the following donations to the Bristol Royal Infirmary Christmas Fund:—
[…] Messrs Lovell and Christmas, Ltd., case of Brown Windsor Soup.

2.6 & 2.7-: From advertisements for Dempster’s, Dumfries, published in the Dumfries and Galloway Standard and Advertiser (Dumfries, Dumfriesshire, Scotland):

2.6-: Of Saturday 17^th February 1945 [page 2, column 3]:

FOR A CHANGE
Try a tin of BROWN WINDSOR SOUP at 6½ d.

2.7-: Of Saturday 22^nd December 1945 [page 2, column 4]:

DO YOU REMEMBER
Batchelor’s Brown Windsor Soup? It was a great favourite in days gone by, and now it’s available again at 6½ d per tin.

Note: The noun Windsor soap designates a kind of scented soap, available in two varieties: white Windsor soap and brown Windsor soap.

Windsor soap was particularly popular in the 19^th century, and was typically scented with spices, citrus and, originally, musk. The earliest versions seem likely to have been white. The brown variety was sometimes regarded as inferior, with the colouring added to it variously explained as an attempt to imitate the colour of soaps which had been aged, and which were considered to be superior (brown Windsor Soap was often sold as old brown Windsor soap), or as an attempt to disguise a greater proportion of a cheaper yellow soap base used in its manufacture.

The earliest occurrence of the noun brown Windsor soap that I have found is from the following advertisement, published in The Derby Mercury (Derby, Derbyshire, England) of Thursday 26^th September 1816 [page 3, column 4]:

Mrs. PARKER,
SILVERSMITH, JEWELLER, & PERFUMER,
Facing the George Inn:

RESPECTFULLY informs her Friends and the Public in general, she has received a large assortment of Perfumery (from Smyth and Nephew, New Bond Street, London):—Best Lavender Water, Honey ditto, Eau de Cologne, Arquebusade, Hungary, Rose, Marlbro’, Huile Antique; their best Tooth, Nail, and Hair Brushes; also their Brown Windsor Soap, fine Ivory Combs, and Brushes; (the above are sold at Smyth’s retail prices.)

The following quotations illustrate how deeply ingrained in British consciousness brown Windsor soap is:

1-: From Risk, a short story by the British author Margery Sharp (1905-1991), published in the Evening Standard (London, England) of Thursday 17^th January 1935 [page 22, column 2]:

I’d already seen her that very afternoon, going solemnly round the Louvre with a catalogue in one hand and a pencil in the other. She had struck me then, even amongst all the Easter-excursion schoolmarms: on a red plush bench in the lounge of the Humide she stood out like—like a bar of brown Windsor in a film-star’s bathroom.

2-: From the Surrey Advertiser and County Times (Guildford, Surrey, England) of Saturday 14^th October 1939 [page 7, column 6]:

Egham’s many contributions to the world in politics, science, letters and other fields were referred to by the Vicar of Egham (the Rev. A. C. Tranter) at a sale of work at the Church Room, on Thursday.
[…] The Vicar said their little town had left its mark on the history of the world. In the field of politics they were noted for Runnemede and the principles of liberty for which we were fighting to-day. In the realm of hygiene they remembered Brown Windsor soap, which played a great part in the cleansing of the world.