"On the brain" is a colloquialism that refers to something that is constantly on one's mind. Some synonyms for this phrase could include "top of mind," "in one's thoughts," "preoccupying," "dominating," or "obsessing over." These synonyms encompass the idea that something is actively occupying one's mental space and attention. It could be a problem, a new project, an upcoming event, a person, or an idea. Whatever the case may be, when something is "on the brain," it can be difficult to focus on other tasks or thoughts. It is important to find ways to address and resolve whatever is on one's mind to avoid distraction and improve productivity.