Crossword clue: Somone who has completely lost it?

The answer "THIN" fits the clue "someone who has completely lost it" because it can refer to a person who has become mentally unstable or fragile. When someone is described as "thin," it often means they are lacking in substance or strength, which can relate to losing their grip on reality. Additionally, the phrase "losing it" can imply that a person is feeling weak or vulnerable, further connecting to the idea of being "thin."