"Gussied up" refers to a person who is dressed up in fancy clothes and accessories, usually for an important event or occasion. Antonyms for this word could be "dressed down," meaning a person who is not wearing any fancy clothes, but rather simple and casual clothing. Another antonym could be "shabby," referring to the poor quality of clothing worn by a person. "Plain" and "unadorned" could also be considered antonyms for gussied up, referring to the absence of any accessories or embellishments. Finally, "uncouth" could be considered an antonym for gussied up, indicating someone who is not polite, refined, or elegant.