The term "spiffed up" refers to the act of dressing up or improving one's appearance. Antonyms for this phrase would include words like sloppy, disheveled, unkempt, or shabby. Instead of putting effort into making oneself look presentable, someone who is sloppy may have messy clothes or unwashed hair. Disheveled people might have tousled or unkempt hair, while those who are unkempt may have untrimmed facial hair or wrinkled clothes. Finally, someone who is shabby may have clothes that are old or worn-out, with holes or stains. All these words are the opposite of "spiffed up," indicating an unimpressive or neglected appearance.