Green’s Dictionary of Slang

you know who n.

also you-know-his-name

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

[UK]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.
[UK] ‘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.
[UK]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.
[US]F. Packard White Moll 166: Is dat straight wot de papers said about youse-know-who gettin’ pinched?
[UK]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.
[US]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.
[Aus]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.