A frat is a fraternity: a social club for male students at a university, especially in the United States. Members often live together in a frat house and throw parties; the names are usually Greek letters. Frat is informal US student talk; the fuller, more careful word is fraternity. Said quickly, it can sound like a party club more than a serious society. Britain does not really have this system, so the word is much less common there. It does not mean “brother” on its own. It is casual, not a rude word.