In the UK there is a prejudice that anything wicked, sexual, and decadent was considered to be of French origin. For instance: «French kiss» «French tickler» etc. Many such expressions date back to 1730-1820, the height of Anglo-French enmity, but some are current and others go back even further." From the «Encyclopedia of Word and Phrase Origins» by Robert Hendrickson (Fact on File, New York, 1997).
:::::: Such a coy phrase as «Excuse my French» or «Pardon my French» goes back to an age-old rivalry between France and the UK and is uttered in an attempt to pass a swearword off as French. The phrase excuse the user of profanity or curses in the presence of those offended by it under the pretense of the words being part of a foreign language.
Possibly, the phrase derives from a literal usage of the exclamation. In the 19th century, when English people used French expressions in conversation they often apologised for it — presumably because many of their listeners (then as now) wouldn't be familiar with the language. The definition cites an example from The Lady's Magazine, 1830:
Bless me, how fat you are grown! — absolutely as round as a ball: — you will soon be as embonpoint (excuse my French) as your poor dear father, the major.
«Embonpoint» is French for 'plumpness'; state of being well-nourished'.
There are a few examples of presumed decadence in reverse. Several expressions in French attempt to link various practices perceived as unsavoury to England, e.g.,
:::::: «l'éducation anglaise» (disciplining children by sexually-tinged spanking)
Ironically, several expressions are used by both the English and the French to describe the same culturally unacceptable habit, but attributing the habit to the other people: e.g.:
:::::: «Taking French leave» (leaving a party or other gathering without taking polite leave of one's host) is referred to in French as «filer à l'anglaise» (literally, «flee English-style», English leave). The intent behind this behaviour is to leave without disturbing the host.
The Oxford English Dictionary records: 'the custom (in the 18th c. prevalent in France and sometimes imitated in England) of going away from a reception, etc. without taking leave of the host or hostess. Hence, jocularly, to take French leave is to go away, or do anything, without permission or notice.' OED states the first recorded usage as: 1771 SMOLLETT Humph. Cl. (1895) 238 'He stole away an Irishman's bride, and took a French leave of me and his master.'
The actual derivation may have its roots in American history during the French and Indian wars. About 140 French soldiers were captured near Lake George in New York and ferried to an island in the lake. The French, knowing the area better than the British, waited until near dawn and quietly waded ashore leaving their captors bewildered on arising. Though its role as such didn't last a day, the island has been named Prison Island.
:::::: Old-fashioned English slang for a condom is «a French letter». Similarly, outmoded French slang for the same thing is «un capot anglais» (literally, an English cap").
:::::: During the 16th century in England (as well as in Italy and much of the Holy Roman Empire), genital herpes was called the «French disease» and «French-sick» was a term for syphilis. These are also considered examples of Francophobia, although in continental Europe they were likely due to foreign (French) armies actually spreading sexually-transmitted diseases.