Skipping one's classes or other academic obligations for an extended period of time in order to pursue other, more socially rewarding activities, such as drinking, golfing, and deep sea fishing. Other acceptable albeit less fratty activities include sleeping, movie-watching, and playing Call of Duty.
Zach showed up to Intro to Accounting for the first two weeks but then decided to go on fratbatical and wasn't seen until the final exam.
an extended period of time one takes off to do fratty things and nothing else.
"hey man, i'm done with these exams. i'm going on a fratbatical."