"Keep one's shirt on" is commonly used as a phrase to suggest self-control or patience, especially during a difficult situation. However, there are other synonyms that one can use to express the same sentiment. One could say "remain calm," "stay composed," "maintain equanimity," or "retain one's cool." These phrases are often used in the face of adversity, frustration, or anger, and remind people to stay level-headed. "Don't get worked up," "don't lose one's temper," and "don't get hot under the collar" are further examples of similar expressions. Using these synonyms demonstrates a level of maturity and composure, and can help to diffuse tense situations.