Green’s Dictionary of Slang

1. to associate with.

1973		J. Lees-Milne Heretics in Love 104: [T]he other girls [...] were stupid, their jokes were feeble, their ‘frattings’ with the sailors were in very poor taste.
1984		‘Derek Raymond’ He Died with His Eyes Open 34: Did she used to frat with anyone in here except Staniland?

2. (US campus) to participate in fraternity parties, events, in order to pick up girls.

1987

Eble Campus Sl. Oct. 3: frat – go to fraternity parties after football games in search of females.