A blend of breakfast and lunch, the noun brunch denotes a late morning meal eaten instead of breakfast and lunch.

It originated, apparently in the late 19^th century, as Oxford University slang and is first recorded in Lunch at Oxford, by Margaret B. Wright, published in The Independent (New York City, New York, USA) of Thursday 22^nd August 1895:

Five students and one stranger sit at table. Behind us a solemn “scout” passed noiselessly backward and forward, so noiselessly that when the lobster salad should give place to cold duck, behold! no scout was there.
“A don in the room below is giving a lunch,” grumbled our host: “he has gobbled the scout.”
Thereupon five Oxford men turned waiters, and, more or less dextrously, deposited salad plates upon the deep window couches and elsewhere, resetting the table from the sideboard.
“Ekker! jolly for the digegger,” they say, with solemn laughter.
What language is this, of ancient peoples and forgotten times?
“‘Ekker’ is exercise, don’t you know? they explain; ‘digegger’ is digestion. When a man walks in the park he takes a ‘pazzer;’ when his people come to see him they are ‘straggers,’ or strangers.”
“Easy enough when you once get the swing,” said the youth known as “Father William,” because of his thinning hair. “Breakfast is ‘brekker’ in the Oxford tongue; when a man makes lunch his first meal of the day it becomes ‘brunch;’ and a tea-dinner at the Union Club is a ‘smug’ at the ‘Ugger.’”

I have discovered a verb use of brunch which predates the earliest quote in the Oxford English Dictionary (OED – online edition, December 2021). It is from The Globe (London, England) of Thursday 16^th July 1896 (the “Lady” refers to the magazine of that name—I have not found the article in question in that magazine):

We have the authority of the “Lady” for a new word. The new word is “brunch,” which is evolved, on the lines laid down by Lewis Carroll, from “breakfast’’ and “lunch.” It means a meal combining the characteristics of the two. He who rises too late for breakfast, and is too hungry to wait for lunch, “brunches.” We suppose also that he who gets up too late for lunch and cannot wait for dinner, “linners,” but the “Lady” does not say so.

The following day, the same newspaper made this correction:

Concerning “brunch” and “brunching,” we have received a letter from a well-informed philologist. The “Lady’” is wrong, he says, in supposing “brunch” to a new word. It is at least two years old, and comes from that repository of new words, Oxford. Moreover, it must not be confounded with “blunch.” He who partakes of a breakfast-lunch combination meal nearer the ordinary breakfast hour than the ordinary luncheon hour, “brunches;” but he who partakes of it nearer the ordinary luncheon hour than the ordinary breakfast hour, “blunches.” The distinction is important.

The South Wales Daily News (Cardiff, Glamorgan, Wales) of Saturday 18^th July of that year described what brunch consisted of:

HOW TO BRUNCH.

To be fashionable nowadays one must, according to the ‘Lady,’ “brunch.” The exercise simply consists of an amalgamated breakfast and lunch, after the manner of the French ‘dejeuner à la fourchette.’ * A proper “brunch” should combine the light dishes of morning with the more substantial fare of afternoon. For liquids there are the inevitable tea and coffee, while those who prefer to follow the robust fashions of our ancestors can have beer or claret. The name is a compound of the nouns “breakfast” and “lunch.”

(* The Dictionnaire de l’Académie française (6^th edition, 1835) defined the now obsolete French expression déjeuner à la fourchette (literally to breakfast with the fork) as “manger de la viande à son déjeuner”, to eat meat at one’s breakfast.)

The earliest use of brunch is often inaccurately attributed to Guy Beringer in the following article, published (according to the above-mentioned edition of the OED) in Hunter’s Weekly (London, England) of Tuesday 5^th November 1895:

BRUNCH: A PLEA.
BY GUY BERINGER.

When one has reached a certain age, and the frivolities of youth have palled, one’s best thoughts are turned in the channel of food. Man’s first study is not man, but meals. Dinner is the climax of each day. You may have your chasse café afterwards, in the shape of theatre, music hall, or social gathering; but it is little more than a digestive. Dinner’s the thing; the hour between seven and eight is worth all the rest put together. A parallel might be drawn between these sixty minutes and the Nuit de Cléopatre; but neither in length nor moral tendency would it be suitable to Hunter’s Weekly. In these hurrying, worrying, and scurrying days the sweets of life are too often overlooked, and, with the sweets, the hors d’œuvre, soups, and entrées. To use a theatrical simile, there is a tendency to regard meals solely as the curtain-raisers of the day’s performances. Who has not whirlwind friends who rush in upon him, exclaiming, “Let’s have a spree to night, old man! We won’t bother about feeding; a chop or steak will about do us.” What a pitiable frame of mind! Not that I am a gourmet. I hate the term. I regard a gourmet simply as a gourmand with a digestion. Excessive daintiness in regard to food is merely a form of effeminacy, and as such is to be deprecated. But there is a happy medium—everything good, plenty of it, variety and selection. On week days these conditions can without difficulty be fulfilled, but Sunday affords a problem for nice examination. All of us have experienced the purgatory of those Sabbatarian early dinners with their Christian beef and concomitant pie. Have we not eaten enough of them? I think so, and would suggest Brunch as a satisfactory substitute. The word Brunch is a corruption of breakfast and lunch, and the meal Brunch is one which combines the tea or coffee, marmalade and kindred features of the former institution with the more solid attributes of the latter. It begins between twelve and half-past and consists in the main of fish and one or two meat courses.
Apart altogether from animal considerations, the arguments in favour of Brunch are incontestable. In the first place it renders early rising not only unnecessary but ridiculous. You get up when the world is warm, or at least, when it is not so cold. You are, therefore, able to prolong your Saturday nights, heedless of that moral “last train”—the fear of the next morning’s reaction. It leaves the station with your usual seat vacant, and many others also unoccupied. If Brunch became general it would be taken off altogether; the Conscience and Care Company, Limited, would run it at a loss. Their receipts on the other days would, however, be correspondingly increased, and they would be able to give their employés a much-needed holiday. The staff has become rather too obstinate and officious of late. That it must be a case of Brunch or morning church I am, of course, aware; but is any busy work-a-day man in a becomingly religious frame of mind after rising between eight and nine o’clock on his only “off” morning? If he went to bed in good time the night before, well and good; but Saturday is Saturday, and will remain so. More especially from seven onwards. To a certain extent I am pleading for Brunch from selfish motives. The world would be kinder and more charitable if my brief were successful. To begin with, Brunch is a hospitable meal; breakfast is not. Eggs and bacon are adapted to solitude; they are consoling, but not exhilarating. They do not stimulate conversation. Brunch, on the contrary, is cheerful, sociable, and inciting. It is talk-compelling. It puts you in a good temper; it makes you satisfied with yourself and your fellow-beings. It sweeps away the worries and cobwebs of the week. The advantages of the suggested innovation are, in short, without number, and I submit it is fully time that the old régime of Sunday breakfast made room for the “new course” of Sunday Brunch.
P.S.—Beer and whiskey are admitted as substitutes for tea and coffee.

Guy Beringer’s article was mentioned in the following from Punch, or the London Charivari (London, England) of Saturday 1^st August 1896:

BRUNCH v. BLUNCH!

According to the ‘Lady,’ to be fashionable nowadays we must “brunch.” Truly an excellent portmanteau word, introduced, by the way, last year, by Mr. Guy Beringer, in the now defunct ‘Hunter’s Weekly,’ and indicating a combined breakfast and lunch. At Oxford, however, two years ago, an important distinction was drawn. The combination-meal, when nearer the usual breakfast hour, is “brunch,” and, when nearer luncheon, is “blunch.” Please don’t forget this.

’Tis the voice of the Bruncher, I heard him complain,
“You have waked me too soon, I must slumber again!
When the clock says it’s twelve, then perhaps I’ll revive,
Meanwhile into bed yet once more let me dive!

The last meal I had was about 3 a.m.;
I’m a writer, so please don’t such habits condemn!
This cross between supper and breakfast I’ll name,
If you’ll let me, a ‘suckfast’—and ‘brupper’’s the same!

Later on, too, a similar mixture I make,
And of ‘five o’clock tinner’ at seven I partake;
The term’s à propos, for the fare is tinned meat,
With tea for ‘ontray’ and lump sugar for sweet.

While the small hours get larger I’m fit as a flea,
The sunrise I’m cheerfully ready to see,
With ‘blunch’ for to-morrow, and no trains to catch,
I don’t need to repose with unseemly despatch.

Beauty sleep is a thing that ne’er troubles my head;
When the cock has done crowing I turn into bed,
Then peacefully dream of the virtues of ‘blunch,’
And, on waking, I rise and indite this to ‘Punch!’”

An English invention, brunch was adopted and adapted in the USA. The Lincolnshire Echo (Lincoln, Lincolnshire, England) of Wednesday 31^st March 1948 described the American brunch:

THIS IS BRUNCH: SUNDAY QUICKIE

Last week’s article on re-organising Sunday led two readers to write asking me the same question. “Can you tell me what the American meal ‘brunch’ really consists of?”
It may interest those of you who are anxious to reduce Sunday work to know that “brunch” was originated to give the American housewife extra free time on Sundays. It is an easily prepared combination of breakfast and lunch and is served any time between eleven and one-thirty.
Nourishing
A seat-down or buffet meal, it contains nourishing food and drink, with only one hot item. A typical “brunch” includes seasonable fresh fruit (grapefruit, oranges and apples), tomato or fruit juice, a variety of cereals, a hot snack and coffee. The hot snack is usually a savoury, a favourite being scrambled eggs with bacon rolls.
The latter are ordinary bread rolls, baked in the oven and served with hot crisp bacon sandwiched between. Eggs baked in cheese sauce are often eaten at “brunch” and so are eggs poached on half muffins and accompanied by corned beef fritters. In winter, soup is added to the menu.