Green’s Dictionary of Slang

used of a person whose name one knows, but prefers not to mention.

1691		N. Ward ‘Poet’s Ramble after Riches’ in Writings (1704) 7: Which if, said I, the News be true, / ’Tis very bad for you know who.
c.1820		‘Sale of a Wife’ in C. Hindley Curiosities of Street Lit. (1871) 40: [title] Sale of a Wife in the Neighbourhood — Mrs. You-Know-Who.
1876		C. Hindley Life and Adventures of a Cheap Jack 86: If you was to go into Mr. You-know-his-name’s shop, in the market-place here.
1920		F. Packard White Moll 166: Is dat straight wot de papers said about youse-know-who gettin’ pinched?
1969		P. Theroux Murder in Mount Holly (1999) 52: The real problem is right here in our midst: the You-Know-Whos [...] they’re all as Red as they are black.
1973		National Rev. 14 Sept. 1007: To get the real inside skinny we shall have to wait until you-know-who’s memoirs tell us what the butler really saw.
2002		S. Maloney Something Fishy (2006) 19: I’m think of having a whip-around, see if I can raise enough for a contract on you-know-who.
2018		(con. 1963) L. Berney November Road 6: ‘She says you work for you-know-who’.