an unauthorised or unannounced absence or departure
The earliest (and most curious) instance of the expression that I could find is in the anonymous novel Benedicta (1741). The heroine is about to get married:
Mrs Butler, who on this extraordinary occasion, had taken French leave of her pillow, was soon at the chamber door, and without taking any apparent notice of her palid countenance, insisted on helping her woman to dress her in bridal splendor.
In the following satirical poem from A Choice Collection of Original Essays, on Various and Entertaining Subjects (London – 1748), the expression is defined as derogatory:
FRENCH LEAVE : A Rondeau
French Leave is a phrase we had often in use,
When one slily elop’d ; nor left coin or excuse:
And oft’, we staid longest, this benefit got;
They were rid of a babbler—for paying his shot.
French Leave, as now taken in Brabant and Flanders,
By the polish’d French troops, and politer commanders,
Is still more refined: For tho’ nothing they pay,
They strip all their hosts—to bear something away.
French Leave is so courteous, ’twill cut a man’s throat,
For claiming his own, or secreting a groat.
French Leave the grave Spaniards in Savoy have learn’d,
And humanely they seize what the peasants have earn’d.
What fools would again such companions receive,
Whilst life could withstand them—if this be French Leave?
In May 1755, Charles Wesley (1707-88), one of the founders of the Methodist Church, also used the expression with negative connotations in a letter that he wrote to his wife about his strained relationship with his sister-in-law:
On my way to Wakefield I met—my good angel and sister. I have done her honour before the people, and behaved, though I say it, very much like a gentleman; only that I took a French leave this morning; that is, left Leeds without telling either her, or her husband. He will follow me quickly with a letter; but I am hardened to causeless reproofs.
However, to take French leave seems to have originally referred to a good-mannered custom, originating in France, of going away from a social gathering without taking leave of the host or hostess. After taking his degree, Joseph Jekyll (1754-1837), future MP, went to France to acquire the French language. During a year’s residence at Blois, he wrote the following to his father on 31st May 1775:
Next to the language, the million etiquettes are the most difficult for a stranger to acquire. They are precise to a degree. For example, I will allow the single circumstance of “taking French leave” (which gains ground even among us at present) is easy and natural. But, on the contrary, I will maintain that there is more formality of compliment in entering one assembly than in taking the rounds for a whole winter at London.
Similarly, in his 1775 edition of Principles of Politeness, and of Knowing the World; by the Late Lord Chesterfield, the Church of England clergyman John Trusler (1735-1820) wrote:
Pulling out your watch in company unasked, either at home or abroad, is a mark of ill-breeding. […] As the taking what is called a French leave was introduced, that on one person’s leaving the company the rest might not be disturbed, looking at your watch does what that piece of politeness was designed to prevent.
The American military expression French furlough, coined after French leave, which appeared in the first half of the 19th century, denotes absence without leave or desertion. It has later been frequently used with reference to the American Civil War, for example by the American author Sandra Dallas (born 1939) in her novel Alice’s Tulips (2000):
There are only two ways Harve can come back to Slatyfork just now—get hurt and come home a cripple or take a French furlough and get sent to the jail.
The French have returned the compliment, since the equivalent of to take French leave is filer à l’anglaise, literally to flee the English way.
An obsolete slang form of this phrase was pisser à l’anglaise (the infinitive pisser means to piss), the image being that one disappears on the pretext of answering a call of nature.