Green’s Dictionary of Slang

an obscenity, notably cunt n., fuck n., shit n. etc; thus six-letter word: bugger n.¹ ; and ten-letter word: cocksucker n. etc.

1936		Mencken Amer. Lang. (4th edn) 301: A fashion for using openly the ancient four-letter words that had maintained an underground life.
1948		New Yorker 23 Oct. 52: In both World War I and World War II, they depended mainly upon a couple of four-letter words that are obscene but not profane.
1951–2		G. Marx letter in Groucho Letters (1967) 91: They cut out two wonderful jokes: To wit: ‘When ever I mentioned work to Jane’s brother he says don’t use that four-letter word in front of my sister’.
1952		J. Jones From Here to Eternity (1998) 275: Angelo seated on his perch grinned as smugly as the parrot who has just four-letter-worded the old maid out of the room in the cartoon.
1969		N. Cohn Awopbop. (1970) 200: He exploded in a rash of four-letter words.
1979		G. Swarthout Skeletons 7: Underneath, the man was a living four-letter word.
1987		L. Redmond Emerald Square 178: I suspected that work was a four letter word in his vocabulary and a dirty one at that.
2003		P. Palms Bankruptcy Solution 18: Is Debt a Four-Letter Word? I think it is.