"to dress up or decorate in a showy way," 1952, American English slang, apparently from Gussy (1940), schoolyard slang name for an overly dressed person, perhaps related to gussie (1901) "effeminate man," and somehow connected to the nickname for Augusta and Augustus.
[late 1940s+; origin unknown; perhaps fr gusset, a triangular insert that might be used to prettify a dress; perhaps fr someone or some place named Augusta]