Another name for the college freshman class; given to the freshman class because in the first year of college, students will usually eat more than usual and not exercise as much, thus gaining weight (usually about 15 pounds)
Sophmore: I just joined the gym... I have to try to work off the Freshman 15.
When upper year collage students have a competition to have sex with 15 freshman girls in a specified amount of time, this could be one night, or one week. The one who reaches the goal, or sleeps with the most about of girls in that specified amount of time, will have bragging rights.
Jenny: "Hey, here's some advice. A lot of guys are doing the freshman 15 this year, so don't get to drunk at that part tonight."