"Smell bad" is a commonly used phrase to describe an unpleasant odor. However, there are numerous synonyms that can be used to convey the same meaning. For instance, the expression "reeking" can be applied to items or surroundings with an extremely repulsive smell. The word "foul" is another apt synonym for smelling bad that can be used for people, objects as well as surroundings. "Stinky" is another choice word that can be used to describe something with an offensive sensation while "putrid" resonates with decaying or rotting smells. Additionally, "noxious," "offensive," "pungent" and "rank" are other effective synonyms that can be used instead of "smell bad".