"Keep your cool" is an idiomatic expression in English meaning, "stay calm." It goes hand in hand with the descriptive phrase, "getting heated," which means becoming angry or overly excited, typically in a negative way. One person might say "keep your cool" to another person if they seem to be getting worked up or excited, although sometimes saying this phrase can have the opposite of the intended effect, if the other person gets offended by it, so it is important to keep this in mind as well. Some people will even say it to themselves as a reminder to remain calm in stressful or difficult situations.
The phrase, "keep your cool," actually makes a lot of sense when the body's physiological response to anger or stress is considered. When a person gets angry or upset, he or she is more likely to breathe quickly and shallowly, and the heart may begin to beat faster. This can actually increase the overall body temperature, particularly in the face; many people will feel that their face becomes red and heated if they are about to yell or become angry. Keeping cool, then, can sometimes be accomplished by slowing the breathing, and consciously trying to avoid escalating a situation.
This is how the phrase, "keep your cool," originated, but it has evolved to mean a number of different things, and is used in a variety of situations. A person who remains cool in a crisis, for instance, does not panic in a stressful situation, but instead stays calm and does what needs to be done. People who stay cool in an argument will not yell or get violent, but will instead speak calmly, even when they are upset. Many people consider this to be a very good personality trait, and will admire people who are able to keep their cool, and think before they speak in a bad situation.
It is important to remember that "keep your cool" is not typically used in positive situations. For instance, if a person gets very excited about good news, he or she would typically not be told "keep your cool," as this could be rude. The phrase is also not used in a literal sense with regard to temperature. Some people simply find it to be a useful mantra to remind themselves to remain calm, and not to get worked up about things that are beyond their control.