The phrase "you know who" is a calque, or loan translation, of the Latin phrase "nomen nescio," often abbreviated as "N.N.," meaning "I do not know the name." It evolved in English as a common colloquial circumlocution, transforming from a literal clause ("you know who [I mean]") into a fixed nominal unit. This transformation exemplifies grammaticalization, where a conversational phrase solidifies into a single lexical item with a specific pragmatic function. Its modern usage leverages shared knowledge between speaker and listener to create an air of secrecy, allusion, or humorous avoidance, directly inheriting the euphemistic and referential logic of its Latin predecessor.

This is a fixed, idiomatic phrase functioning as a compound noun. It is not productively formed from separable morphemes in modern English but is a direct combination of common English words.