Someone who doesn't let their emotions show is often referred to as "stoic" or "emotionally reserved." The term "stoic" comes from the ancient philosophy of Stoicism, which emphasizes self-control and the suppression of emotions as a means of achieving tranquility and rationality. People who are stoic tend to remain composed and calm even in challenging or emotional situations, and they may not openly express their feelings. However, it's important to note that everyone experiences and expresses emotions differently, and being stoic doesn't necessarily mean that someone lacks emotions altogether; they may simply prefer not to display them openly.