adj: used to describe actions, events, and attire that represent the fraternity.
If you don't go to an Ivy League school, or didn't attend a New England prep school, chances are, you are not fratty. Sorry.
Brooks Brothers, Godlike Keg Stands, Thomas Pink, and Vineyard Vines are all fratty.
adj. used to describe a person as being a closet homesexual, while at the same time wasting their parents money on "straight" clothes they noticed on a half-naked male model at Abercrombie and Fitch (Polo and Lacoste are another two companies who market to these "closets") Also used to describe specific items of clothing that everyone around you also owns, and also got turned on by when they saw their male friend wearing their shirt "the next morning."
"Dude, we are so fratty!... But did we do it last night after our frat party? Well, it doesn't matter.. we clearly aren't gay since our shirts are so fratty."
"Yeah, but I love to see your sack hanging from that fratty shirt.. but I should hush up since we are supposed to act straight"