Slick up has a few different meanings, including to tidy or clean up, to dress up, or to make something more attractive or appealing. Some antonyms for slick up might include words that convey a lack of attention to appearance or cleanliness, such as neglect, squalor, or dirtiness. Other antonyms might focus on a more casual, natural look, such as relaxed, informal, or understated. Still others might highlight a lack of effort or concern about appearance, such as unkempt, disheveled, or slovenly. Ultimately, the choice of antonym will depend on the context and the desired effect of the writing or speech.