To gussy up is to decorate something or dress up for a special occasion. You might want to gussy up your living room before throwing that surprise party this weekend!
When you want to make something more fancy or festive, you gussy it up. You might gussy up your house — touching up the paint and organizing the clutter — before trying to sell it. And you might gussy up for the first day of school, styling your hair in an extra fancy way and wearing your favorite outfit. One guess about the origin of this phrase links it to an old Australian nickname for a man who cares too much about his appearance: a gussie.