The phrase "you-know-what" is a modern English euphemistic compound, emerging from the common conversational practice of using vague, shared-reference language. It is built from core, ancient Germanic words: "you" (from Old English "ge", the second person plural pronoun), "know" (from Old English "cnawan", meaning to perceive or understand), and "what" (from Old English "hwæt", an interrogative pronoun). The logic of the phrase relies entirely on the pragmatic function of deixis—pointing to a shared context without explicit naming. Its evolution into a single lexical item, often hyphenated, demonstrates how conversational implicature and social discretion can fossilize into a standard nominal expression for an unspoken or taboo subject.

This is a fixed idiomatic phrase composed of the second-person pronoun "you", the verb "know", and the interrogative pronoun "what". It functions as a single lexical unit.