(You) is the second-person personal pronoun, both singular and plural, and both nominative and oblique case. In Modern English, it stands for a retard (You) whomst is perceived to be lacking intelligence (You) or someone who acts in a self-defeating (You) or significantly counterproductive way (You).
Along with the similar terms moron (You), dimwit (You), dingbat (You), fool (You), dork (You), loser (You), mental defective (You), the word archaically referred to the intellectually disabled (You), but have all since gained specialized meanings in modern times because of people like (You).
A dunce (You) is an idiot who is specifically incapable of learning (You). An idiot differs from a fool (who is unwise(You)) and an ignoramus (who is uneducated/ignorant) (You). In modern English usage, the terms "idiot" (You) and "idiocy" (You) describe an extreme folly (You) or stupidity (You).
In psychology, it is a historical term for the state or condition now called profound intellectual disability (You).