The informal and derogatory British-English name Johnny Crapaud, and its variants, are used to personify France or the French people, or to designate a typical Frenchman.

This name is composed of:
– Johnny, a pet form of John, which is used, with modifying word, to designate a person, especially a man, of the type, group, profession, etc., specified—cf. for example, in British English, Onion Johnny, Johnny Foreigner and Johnny Arab;
– the French noun crapaud, denoting a toad—cf. the use of the noun frog, which has long been a general term of abuse, as an informal and derogatory appellation for a French person.

The name Johnny Crapaud, and its variants, seem to have been coined by British sailors during the Napoleonic Wars (1800-1815).

Perhaps with reference to crap, denoting excrement, faeces, this name first occurs as Johnny Crap in a text dating from 1805, by John Brown (1754-1832), minister of the Secession church—as edited by Henry George Thursfield (1882-1963) and published in Five Naval Journals: 1789-1817 (London: Printed for the Navy Records Society, 1951):

we engaged five Ships at one time but they would have Sunk us only for the timmera [Temeraire?] took the firy edge off us  the repaiting Frigate Could not see us for fire and smoke from 12 oclock until two they thought we were sunk but instead of that we were giving Johnny Craps their breakfast

The name then occurs as Johnny Crapaud in the following passage from The Cruise; A Poetical Sketch, in Eight Cantos. By a Naval Officer (London: Printed for J. Hatchard, 1808):

Yet, tho’ to make all sail we thirst,
Johnny Crapaud * had seen us first.
* A very common nickname for the French among our Sailors.

These are two other early occurrences of Johnny Crapaud and variants:

1-: From Travels in England, France, Spain, and the Barbary States, in the years 1813—14 and 15 (New York: Published by Kirk and Mercein – London: Published by John Miller – 1819), by the U.S. diplomat Mordecai Manuel Noah (1785-1851):

It occurred to me during the representation, that a singular scene would ensue, if in the most interesting part of the performance notice should be given, that the French fleet, which lay in sight at anchor, had weighed and were standing out to give us battle. I imagined the scene of confusion that would take place.—The theatre and scenery demolished at one rude crash—drums beating—marines under arms—tompions out of guns—powder monkeys at work—doctors preparing lint and bandages—and Ephraim Smooth, with the rest of the Dramatis Personæ, ramming down grape and langrage, with an expedition which left no time even to divest themselves of their stage dresses. If ever a battle was a desirable object to a non-combatant, I should have been content and well pleased to have seen one at that identical period. But there was no danger; Johnny Crapaud, as the officers familiarly and rather uncourteously termed the French, were snug in port, and probably at the very moment were representing a grand melo-drama, a ballet, or a tragedy of Racine, on board their own squadron.

2-: From the Chester Chronicle. And Cheshire and North Wales Advertiser (Chester, Cheshire, England) of Friday 5^th February 1819:

BATTLE OF WATERLOO.

It is amusing to observe, the force of a habit of presumption in the French character:—An Englishman, after being beaten, would scrupulously avoid enlarging upon the uncalculating confidence with which he went to battle. So natural, however, is that style to a Johnny Crapeau, (Toad) that three years after his signal defeat, we still have the following sentence in General Gourgaud’s campaign of 1815, written at St. Helena, under the immediate eye, if not from the actual dictation of the fallen Corsican. In short, virtually, a history by Napoleon Bonaparte himself, of his own final discomfiture: [&c.].

The name Johnny Crapaud and variants were often contrasted with, sometimes likened to, John Bull, used to personify England or the English people, or to designate a typical Englishman. For example, the following is from a letter to the Editor, published in The Sun (London, England) of Wednesday 22^nd March 1815:

Sir, I call the attention of the Legislature to ALL the Taxes that affect houses, which none of us can do without, any more that we can without bread.
This is the popular topic that has succeeded to that of the Property Tax and the Corn Bill, and will last some time, if not lost in the contemplation of that wicked rebellion in France, brought about by our neighbour Johnny Crapaud, from the same restless spirit that now actuates John Bull.

Likewise, the following is from The Morning Herald (London, England) of Thursday 25^th January 1827:

AFFAIR OF HONOUR IN HIGH LIFE.
(FROM A CONTEMPORARY.)

As a matter, that lately has drawn much attention,
’Twixt John Bull, Esquire, and one Monsieur Crapaud,
Is still undecided, we think right to mention
The leading particulars, far as we know.

The dispute (like a duel, some twenty years back*)
Began about dogs,—in a way much the same;—
Old Crapaud’s dog, Scamp, having made an attack
On the dog of Squire Bull, Constitution by name.

Constitution, a sprightly young pup, thorough bred,
Having “Bull” on his collar, was let to run free;
While Scamp in a leash by his master was led,
Like those lap-dogs we oft in the Tuileries see.

No sooner had Scamp, by a snap at the pup,
In his old lurcher fashion, committed the breach,
Than the blood of Squire Bull in an instant was up,
And he made (as report says) a dev’l of a speech;—

In which, nothing sparing his high-pepper’d phrases,
He curs’d and he foam’d at the cur-dog’s invasion;—
Talk’d much about “Æolus”—“fœderis casus”—
And other Dog-Latin, befitting th’ occasion;

And said (though for this this he soon made an excuse),
That, if the Mounseer didn’t pull in his tether,
He’d let all the bull-dogs in Christendom loose,
And demolish both Scamp and his master together!

On hearing this speech, Crapaud made a low bow—
Acknowledg’d, like Launce, that his dog was a bad one;
And vow’d, if he dar’d but to utter “bow-wow,”
Mr. Bull might, at once, hang him up as a mad one.

This was all mighty fine—but, to folks who stood near,
It was plain that, whatever old Crapaud might speak,
He and Scamp were alike, for they saw that Mounseer,
All the while that he spoke, had his tongue in his cheek.

And, still as his mongrel return’d to the fray,
In watching his hands, they perceiv’d that, with one,
He affected to draw the base lurcher away,
While, with t’other, he slily kept patting him on.

Thus things stand at present—old Crapaud still bowing,
Whenever they meet, like a well-behav’d frog,
But still, in the midst of this blarney, allowing
His Scamp to keep worrying Mr. Bull’s dog.

Meanwhile, their two families feel in a hobble,—
Both Crapauds and Bulls being sick of disasters,
And fearing, poor folks, as this beautiful squabble
Began with the curs, it may end with the masters.

* Between Colonel Montgomery and Captain M‘Namara.

And the following is from Sketches and Recollections of the West Indies. By a Resident (London: Published by Smith, Elder, & Co., 1828):

One of the greatest annoyances in the West Indies, is the vile croaking of the lizards, wood-slugs,* and frogs.
* Crapauds. These are as large as chickens, and, when fricasseed, are nearly as tender. They are relished much by the French, and custom reconciles even John Bull himself to Monsieur Crapaud, on the dinner table. After supper, he is insufferable.