Shortened form of fraternity, college organization. (Often used as a noun modifier.)

plural of frat

(US) A member of a fraternity in a college or university

plural of frat boy

(US) A building in which the members of a fraternity reside or meet, especially one located on or near the campus of a college or university.

plural of frat house

(slang) Literature perceived to appeal to, or be marketed at, young men or bros, typically concerning violence, adventure, war, and military themes.

(informal) A group of comedy actors who have appeared together in many of the highest-grossing comedy movies since the mid-1990s.

A monk. / A frater house. / A comrade.

(intransitive) To associate with others in a brotherly or friendly manner. / (intransitive) To associate as friends with an enemy, in violation of duty.