"Blow your top" is a colorful idiomatic expression that is used to indicate that an individual is extremely upset and is about to or has already lost his or her temper. The imagery connected with losing the temper focuses on the vision of the top of someone’s head actually coming off, due to the buildup of the internal pressure caused by the anger. Typically, the term is reserved for use when a particular event or subject inspires very deep negative feelings, making it more difficult to remain composed and logical in terms of how the situation is approached.
Unlike some other idioms that are used in present tense only, it is not unusual for "blow your top" to be used as not only something that is happening now but something that may happen in the very near future. For example, someone may attempt to calm someone down who is obviously about to lose his or her temper by encouraging them to step away before “you blow your top.” In this use of the saying, the idea is to prevent what would likely be a very negative event from taking place by urging some temporary distance from the issue that is causing so much distress. Once the individual who is experiencing the extreme anger has a chance to regain some composure, it may be possible to address the underlying issue with a more balanced view and work toward a solution that is equitable for everyone concerned.
At other times, "blow your top" refers to something that is happening this very minute. When this is the case, the individual who is losing his or her temper is in the process of expressing a great deal of anger, with those expressions including a release of verbal comments that may or may not be logical or even understandable. In some cases, the release of anger may also include some type of physical activity such as throwing a glass into a fireplace or tossing a chair across the room. If left unchecked, this release of anger may take a more violent turn that leads to physical altercations between two or more individuals.
In spite of the comical image that is sometimes associated with blowing a top, the idiom normally refers to a situation that is anything but funny. Excessive anger rarely solves any issue and may in fact only make things worse. For this reason, finding ways to not blow your top, but to channel that anger into some more constructive process is often a better way of dealing with the situation.