Acting with decorum is a behavior that is characterized by dignity, civility, and respectability. On the other hand, antonyms for the phrase can include acting indecently, coarsely, or rudely. These terms describe behaviors that lack courtesy, etiquette, and good manners. Other antonyms can include acting crudely, tactlessly, or boorishly, which are considered inappropriate under any circumstances. A person who behaves in such a manner may be seen as uncivilized or uncouth. Therefore, it is essential to strive to act with decorum in all situations to earn respect and to present oneself in the best possible manner.