The name in the Harry Potter wizarding world for Lord Voldemort. He has also been called "He Who Must Not Be Named", "Lord Thingy", and other appellations of the sort.
Ron: "Hey, Hermione, I heard that You-Know-Who is trying to find Harry. D'you think he wants to kill him?"
A person, usually an acquaintance, who you find completely detestable. The mere mention of their name is enough to fill you with a feeling of dread, while an encounter with this person inevitably leaves a cloud of misery hanging over an otherwise pleasant day. This term is derived from a common reference to Lord Voldemort from J.K. Rowling's Harry Potter series.
"I bumped into You-Know-Who the other day...that basically ruined my week."