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The following definition of the expression brain rot is from the Oxford English Dictionary (online edition, June 2025):

A perceived loss of intelligence or critical thinking skills, especially (in later use) as attributed to the overconsumption of unchallenging or inane content or material. Now also: content or material that is perceived to have this effect.
In recent use particularly associated with the overconsumption of such content posted online.

The expression brain rot occurs, for example, in the following from Continuing the quest for human redemption, by Shane Brown, published in The Dispatch · The Rock Island Argus (Moline, Illinois, USA) of Friday 4^th October 2024 [page A7, column 1]:

Are we SO gullible that we now believe any bit of random nonsense we read online and run with it as fact? I keep seeing evidence of our collective brain-rot everywhere. Is anyone still empowered by free thought? Is anyone still a good person? Or have we fallen down so many rabbit holes of stupidity and debauchery that humanity itself has jumped the shark forevermore?

These are, in chronological order, the earliest occurrences of the expression brain rot that I have found:

1-: From Walden; Or, Life in the Woods (Boston: Ticknor and Fields, 1854), by the U.S. author and naturalist Henry David Thoreau (1817-1862) [page 348]—Thoreau apparently coined brain rot after potato rot:

Why level downward to our dullest perception always, and praise that as common sense? The commonest sense is the sense of men asleep, which they express by snoring. Sometimes we are inclined to class those who are once-and-a-half witted with the half-witted, because we appreciate only a third part of their wit. Some would find fault with the morning-red, if they ever got up early enough. “They pretend,” as I hear, “that the verses of Kabir have four different senses; illusion, spirit, intellect, and the exoteric doctrine of the Vedas;” but in this part of the world it is considered a ground for complaint if a man’s writings admit of more than one interpretation. While England endeavors to cure the potato-rot, will not any endeavor to cure the brain-rot, which prevails so much more widely and fatally?

2-: From Our Feuilleton. On Saturday Night.—II. With a Set of Teaspoons, by ‘J. G.’, published in the Illustrated Times (London, England) of Saturday 10^th December 1864 [page 379, column 2]:

Mr. Mookov delivered a longish address on the advantages of “total abstinence” and the frightful results of beer-drinking […]; after bringing both his fists to the table with a tremendous bang, he consigned the drunkard’s soul to everlasting torment, wiped his perspiring brow with his handkerchief, and said, in the blandest of voices,
“And now, after business, let seek pleasure. Let us show the loathsome drunkard, as we have shown him before, that we can be merry as well as wise, that we can bandy the jest of abstinence, and laugh the temperate laugh and sing the soul-enlivening song with as loud a voice and as hearty as he, hiccuppy with strong drink, drivels forth his blasphemous legends in praise of blood-poison—in praise of brain-rot—in praise of a shaven head and a madman’s rattling chain! Brothers, we will have a song. What shall it be?”

3-: From The New York Herald (New York City, New York, USA) of Sunday 20^th September 1868 [page 6, column 3]:

Menken and the Bohemians.

There is a great pother ¹ making just now in certain portions of the press over the genius and other extraordinary qualities of the recently deceased actress, Adah Isaacs Menken ² […]. All this is the work of those irrepressible fellows, the Bohemians. […]
[…] There is no power more potent for drawing good enough women from the dull but better course of domestic life than the illusory brilliancy cast around the career of such a woman as this latest victim of Bohemian debauchery and brain rot.

¹ The noun pother means: noise, tumult.
² Adah Isaacs Menken (1835-1868) was a U.S. actress.

4-: From The New-Orleans Times (New Orleans, Louisiana, USA) of Wednesday 19^th January 1870 [page 2, column 1]:

The New York Commonwealth refers to some religious verses which appeared in this paper as a specimen of Southern “brain-rot.” As it happens the verses in question were written by a Northern lady whose poetical productions have been praised, and that highly, in Northern papers.

5-: From The Church in Wales: Its Decline, and the Rise of the National Free Churches. No. 2., published in The Cardiff Times. South Wales, Monmouthshire, and Western Counties Advertiser (Cardiff, Glamorgan, Wales) of Saturday 14^th May 1870 [page 5, column 4]—the author perhaps coined brain rot after dry rot:

A more edifying spectacle was not to be seen than the hue and cry, the diligent use of the pastoral staff, the invocation of the secular arm, by the bishops and clergy and magistrates and gentry, when a poor shepherd lost his way as Wroth, and Erbery, and Cradock did, and was found wandering in the valleys and on the mountains after sheep that had no pasture. John Penry had gone so mad after the fashion, that he had to be despatched, for although all over the land there were parishes, and in the parishes parish churches, and in the parish churches a Bible for the use of the parishioners, and in the pulpit, at least now and then, an ordained priest, or his curate, yet he had talked of evangalising [sic] his “dear country of Wales” by what amounted to lay agency and voluntary contributions! And now W. Wroth, the rector of Llanvaches, and W. Erbery, of St. Mary’s, Cardiff, and Walter Cradock, his curate, have caught the like distemper, be it dry rot or brain rot, and will infect the sheep with it.