Mat: What's a brainer?
Someone that is very smart and seems to excel at everything. The type that always seems to have some new project that requires exceptional intelligence to even plan, let alone implement.
The type a student will go to when they need serious help with a test/exam or project for a class they're doing poorly in. Although some may explain said subject in a way that that will have the person requiring help saying ‘in English please’, once they’re able to get on the same level with the person, a brainer can be a very valuable study buddy.
Person #2: Really? That lucky dog; Jimmy is a major brainer. With him as his partner, Billy is practically guaranteed an A!