The word "spiffs up" means to make something look smarter or neater. Therefore, the antonym of spiffs up would be to make something look shabbier or unkempt. Other antonyms for spiffs up could be to make something look messier, disheveled, or rumpled. Alternatively, you may want to make something look more casual or untidy. You could also use antonyms such as "deconstruct," "scuff," "scramble," or "disorder" to describe the opposite of spiffs up. Whatever the antonym, it represents the opposite of transforming something into a slick, polished appearance.