Green’s Dictionary of Slang

fratty adj.

(US campus) pertaining to fraternity life.

1996		Eble Sl. and Sociability 33: Another frequently used suffix is adjective-forming y [...] College slang has dorky, fratty, freaky, geeky, grooty, lunchy, spacey, squirrelly, and others.
2003		Eble Campus Sl. Nov. 3: fratty – epitomizing fraternity behavior and dress. Also fratastic, fratstar.
2004		Eble Campus Sl. Apr. 3: fratastic [...] Those croakies (holders for sunglasses) are so fratastic.
2014		C. Eble (ed.) UNC-CH Campus Sl. Spring 2014 Fall .

In compounds

fratty-bagger (n.) (also bag, bagger) [bags n.² (1), i.e. the style of baggy trousers worn]

(US campus) a stereotypical fraternity member.

1973		Eble Campus Sl. Mar. 2: fratty-bagger – someone who [...] loves fraternity life: Fratty baggers always dress the same: corduroy jackets, crew-neck sweaters, and topsider shoes.
1977		Eble Campus Sl. Fall 1: bagger – a member of a fraternity.
1980		Eble Campus Sl. Fall 3: fratty-bagger – fraternity member, usually dressed in Izod shirts and baggy khakis.
1982		Eble Campus Sl. Fall 1: bag – a fraternity member.
1986		Eble Campus Sl. Mar. 1: bagger – man who wears his boxer shorts so they hang out below his regular shorts.
1996		Eble Sl. and Sociability 64: ‘A stereotypical fraternity member’ has for years been called a fratty-bagger or a bagger because of a style of pants once fashionable.