Especially in the phrase to meet one’s Waterloo, the noun Waterloo designates a decisive or crushing defeat.

The allusion is to the last major battle of the Napoleonic Wars (1803-15), fought outside the village of Waterloo, near Brussels, on Sunday 18^th June 1815, in which Napoléon I * was decisively and finally defeated.

* Napoléon Bonaparte (1769-1821), Emperor of the French as Napoléon I from 1804 to 1815.

It was not long after the battle that Waterloo came to be used figuratively: the English poet George Gordon Byron (1788-1824) wrote the following in a letter to the Irish poet Thomas Moore (1779-1852), dated Venice, Sunday 17^th November 1816—as published in Letters and Journals of Lord Byron: With Notices of his Life, by Thomas Moore (Frankfurt on the Main: Printed by and for H. L. Brönner, 1830) [page 309]:

By way of divertissement, I am studying daily, at an Armenian monastery, the Armenian language. I found that my mind wanted something craggy to break upon; and this—as the most difficult thing I could discover here for an amusement—I have chosen, to torture me into attention. […] Four years ago the French instituted an Armenian professorship. Twenty pupils presented themselves on Monday morning, full of noble ardour, ingenious youth, and impregnable industry. They persevered, with a courage worthy of the nation and of universal conquest, till Thursday; when fifteen of the twenty succumbed to the six-and-twentieth letter of the alphabet. It is, to be sure, a Waterloo of an Alphabet—that must be said for them.

The noun Waterloo soon came to be used in phrases such as to meet (with) a Waterloo defeat and to sustain a Waterloo defeat—the earliest occurrences that I have found are as follows, in chronological order:

1-: From the Nashville Whig (Nashville, Tennessee, USA) of Monday 29^th November 1824 [Vol. 13, No. 16, page 3, column 1]:

Governor of New-York.—It is now reduced to a certainty that Dewitt Clinton is elected Governor of New-York, by a large majority over Col. Young. Mr. Noah very humorously acknowledges that his party has received a Waterloo defeat.

2-: From the Missouri Intelligencer and Boon’s Lick Advertiser (Fayette, Missouri, USA) of Friday 15^th August 1828 [Vol. 10, No. 4, page 3, column 1]:

“Spread the news abroad!”

The friends of the Administration have gained a great victory in Louisiana. The entire administration ticket in that state has succeeded. The Nashville Republican (a Jackson paper) in endeavoring to put the best face on the matter, says, “We have sustained a Waterloo defeat in New Orleans—routed, beaten and overthrown. Livingston has lost his election, and the full Administration ticket for members of the Legislature prevailed.”

3-: From Masonry and the Church, published in the North Star (Danville, Vermont, USA) of Tuesday 26^th April 1831 [Vol. 25, No. 15, page 3, column 3]:

Slade D. Brown, the masonic bully for supervisor, and Soloman S. Cowan, the cable-towed candidate for Justice of the Peace, have both met with a Waterloo rout; cut up and defeated, horse and foot, they have gone to the blessed shades of private life, whence they will not soon be recalled.

4-: From the Western Statesman (Lawrenceburg, Indiana, USA) of Friday 31^st August 1832 [Vol. 3, No. 25, page 3, column 2]:

We have heard it said before that rabidness caused animals to go blind, but surely we never considered Mr. Culley really blind, but wilfully, although he had been frothing at the mouth for sometime, “like a chafed boar.” But the awful reality now presents itself, that the relentless hand of adversity is about to level one of Indiana’s proudest monuments—which is brought about by his “intruding convictions” of meeting a “Waterloo defeat,” in November.

5-: From the New-York Evening Post (New York City, New York, USA) of Tuesday 16^th October 1832 [No. 9,412, page 2, column 3]:

Ohio Election.—We have just been favored with the perusal of a letter from a gentleman in Columbia County, Ohio, dated Oct. 10^th, from which we make the following extract.
“The election for Governor, members of Congress, and members of the State legislatare [sic], took place yesterday in this State, the result of which is, that this county is exceedingly favorable to the Jackson cause, for our opponents have met with a Waterloo defeat.

The earliest occurrences that I have found of the phrase to meet one’s Waterloo, also to meet with a Waterloo and to meet the Waterloo of, are as follows, in chronological order:

1-: From a correspondence from Paris, dated Sunday 16^th December 1832, published in The Courier (London, England) of Tuesday 18^th December 1832 [No. 12,909, page 2, column 1]—French “a été le mot lugubre de” translates as “was the gloomy word of”:

In France, notwithstanding her two revolutions, there exist statesmen who, under the influence of the traditions of the empire, can conceive no other mode of Government than material force. Napoleon, in defiance of the prestige of his Italian and Egyptian campaigns, was not pardoned the 18^th Brumaire, and all the usurpations that ensued, before he enveloped them with the imposing words of Marengo, Austerlitz, and Jena; and even then, as the Minister Thiers lately observed, it ought not to be forgotten that the last of these imposing words a eté [sic] le mot lugubre de Waterloo. But in these days Napolean [sic] himself, in defiance of the halo of his renown, would struggle in vain against public opinion, and would meet with a Waterloo in the interior without the aid of a foreign coalition.

2-: From the Public Ledger (Philadelphia, Pennsylvania, USA) of Thursday 25^th January 1838 [Vol. 4, No. 104, page 2, column 2]:

Burglary.—On Tuesday night, the yard gate and back door of a house No. 12 Bread st., were broken open, and the house robbed of a considerable quantity of money, which was in a box and pocket book. The villains did not succeed in getting into the store. The sufferer by this robbery is a poor woman, who keeps a huckster stand, and depends upon the income thus derived, to support herself and family. These back-door scoundrels must be relations (perhaps twin brothers) to the gang who attempted a robbery in Callowhill street yesterday morning. Their modus operandi is the same. Let citizens keep a bright look-out, and the rascals will soon “meet their Waterloo,” and St. Helena too.

3-: From the Public Ledger (Philadelphia, Pennsylvania, USA) of Saturday 8^th December 1838 [Vol. 6, No. 63, page 4, column 1]:

Capture of a Counterfeiter.—We learn from New York papers, of the arrest, in that city, of Smith Davis, somewhat celebrated among his subjects as the “King of the Koniackers,” or the “Napoleon of the West,” but known to the police officers as a villain who has for some twenty years past been a regular manufacturer and seller of counterfeit money. […] Owing to the system of burying the plates in some remote place, none have been discovered from which the counterfeits were printed. His imperial highness has at length, however, met his Waterloo, and will most probably find a St. Helena, as he is arrested under circumstances which can hardly fail to secure his conviction.

4-: From New York Fires and Firemen, a correspondence dated New York City, Saturday 16^th February 1839, published in The Madisonian (Washington, District of Columbia, USA) of Tuesday 19^th February 1839 [Vol. 2, No. 62, page 3, column 6]:
—Note: At the time of the Great Fire of New York City, on Wednesday 16^th December 1835, major water sources, including the East River and the Hudson River, were frozen, which hampered the fire-fighting efforts:

Personal hazard is with him [i.e., the New York fireman] a play thing. He sports with danger and laughs at peril. Although, occasionally, the hero of his own tale, yet he is no egotist. He attracts your attention to the scenes in which his own prowess is conspicuous; yet the whole is related as a matter of course, and not events to excite either wonder or admiration. The victor is an hundred fights, yet like the throne-dispensor of Europe, he met his Waterloo at the great fire of December, 1835.
Deserted by his natural ally, and assailed by adverse elements, he was fain to resign his watery sceptre to the prowess of the ice king. But unlike the Corsican conqueror, no St. Helena chains his energies, and the defeat of the hour has been amply avenged.

5-: From an article about the suspension of specie payments in Philadelphia, Pennsylvania, USA, published in The Charleston Mercury (Charleston, South Carolina, USA) of Wednesday 16^th October 1839 [Vol. 28, No. 4,881, page 2, column 3]—reprinted from the Pennsylvanian:

The New York Banks were wiser, and hence it was that when they were cured and ready to resume after the lapse of a few months, they were so furiously threatened by what is called the bank press of Philadelphia, and told if they dared to come up from “their Elba” of unredeemable paper, they should meet the “Waterloo” of irretrievable bankruptcy, and hence it is that the New York institutions, are now unwilling to accompany us to the “St. Helena” of suspension, on which barren spot an effort is making to induce us to sit contentedly.