Fastuous is an adjective that means ostentatious, pretentious or vulgarly showy. Its antonyms are words that denote the opposite of these qualities. Some suitable antonyms for fastuous include humble, unpretentious, simple, restrained, understated, modest, unassuming, genuine, and unostentatious. These antonyms suggest qualities that are the opposite of self-importance, lack of taste, and excessive showiness. For instance, a humble person does not show off his wealth or accomplishments, whereas an ostentatious person flaunts them. Similarly, an unpretentious design is restrained and functional, while a fastuous design is showy and artificial. Thus, using these antonyms can help one convey a more moderate, tasteful, or authentic message.