Green’s Dictionary of Slang

French letter n.

also letter
[French adj. (2) + SE letter; accepted as SE since 1950s]

a contraceptive sheath; rare vars. are American/Italian/Spanish letter.

Age (London) 4 Sept. 296: By the way, how will our friend Tom [Duncombe, a well-known womaniser] like being styled ‘Superintendant of French Letters Department?
[UK] ‘Toasts’ in Ri-tum Ti-tum Songster 47: Cupid’s boarding school, where the naked truth is taught, and the ladies hate French letters.
[UK] ‘Nursery Rhymes’ in Pearl 4 Oct. 33: A president called Gambetta / Once used an imperfect French Letter.
[UK]Randiana 52: Now all you young ladies take warning had better / [...] / When you treat John make him wear a French letter. [Ibid.] 115: I surreptitiously pulled off the letter and let my John Thomas approach his lair au naturel [...] flesh is cent. per cent. better than a nasty gutta percha cover.
[UK]‘Walter’ My Secret Life (1966) II 371: I began also to use French letters, for reasons she advised me to do so.
[Ire]Joyce Ulysses 354: French letter still in my pocketbook. Cause of half the trouble. But might happen sometime, I don’t think.
[UK]J. Franklyn This Gutter Life 157: French letters. He supplies the prostitutes for miles round.
[UK]E. Rutherford 19 Oct. diary in Garfield Our Hidden Lives (2004) 116: The woman who cleans corridors, foyers, lifts, etc. to these flats was going off the deep end terribly yesterday. She had just swept two French letters out of the phone booth in the foyer.
[Ire]J.P. Donleavy Ginger Man (1958) 360: French letters floating out to sea. Ought to auction them off in Dublin.
[UK]N. Dunn Up the Junction 39: Anyone want any cheap underwear [...] French letters, boxing gloves?
[UK](con. WWII) B. Aldiss Soldier Erect 67: I [...] rolled the french letter I had brought down the length of my prick.
J. Atkins Sex in Lit. IV 188: In the end he couldn’t get it in so he pulled the letter off and asked the girl to masturbate him.
[Aus]N. Keesing Lily on the Dustbin 27: Ah, that’s better, as the old woman said when she removed the french letter that had been there since Armistice Day!
[Aus]M.B. ‘Chopper’ Read How to Shoot Friends 113: You can bet Paris to a French letter she wasn’t in love with any of them.
[UK]A. Sillitoe Birthday 7: He learned enough from a mate in the factory to call at the chemist’s once a week and provide himself with an adequate supply of french letters.