The phrase "get a grip" is another way of saying "calm down" in the sense of taking control of one's feelings. Sometimes a person also uses it to mean "get back to reality" when the person he or she is speaking to is saying things that seem unrealistic or behaving in an irrational manner. The phrase does not mean to actually take a physical grip of something. Instead, it is an idiom — a phrase used to express a thought using words that are not meant to be taken literally.
If a person wants to calm another party, he may simply say "calm down" or even "relax." He does not have to use this type of wording to express his thought, however. There are many ways he can choose to say the same thing, and one of them is the phrase "get a grip." It usually means to calm down or get in better control of one's emotions or thoughts. In some regions, the phrase is commonly used in this manner.
Some people also use the phrase "get a grip" when talking to someone who is expressing irrational thoughts or behaving in a problematic manner. In such a case, the phrase may mean to stop the irrational talk or behavior. In this manner, it is used similarly to the expression "get real," which also means to come back to reality.
The phrase "get a grip" is an idiom, which means it is an expressive but non-literal phrase. It is commonly used in some regions, especially in the United States. As such, many people who grew up in the United States are familiar with it and understand what it means without any explanation. People who were born or live in places where the idiom is uncommon may need explanation, however, as they are unlikely to understand it and may try to interpret it literally.
Generally, "get a grip" and other idioms are considered acceptable for speaking to others in a casual manner. For example, a person may use the phrase when speaking to a friend or family member or even when sending emails and text messages. He is unlikely, however, to use the phrase in an academic paper, when speaking to his boss, or during a job interview.
Sometimes this phrase can have a negative tone as well. This is due to the fact that it is often used when a person believes another is behaving or speaking in a crazy or irrational manner. The person who hears this phrase may not always view his own behavior in the same manner and may view its use as negative or even insulting.