"The Duff" is a slang term that stands for "Designated Ugly Fat Friend." It refers to a person in a social group who is perceived as less attractive or less desirable compared to their friends. This term often highlights the dynamics of social interactions and can carry negative connotations, as it reduces a person to their physical appearance. The concept gained popularity through the 2015 film titled "The Duff," which centers around a high school student who discovers she is the Duff among her more attractive friends. The movie explores themes of self-acceptance and the importance of inner beauty, challenging the stereotypes associated with the term.