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 Freshman15.
When a first year college student comes back the following year and appears to have gained about 15 pounds from eating too much and not enough exercise.
Did you see Ashley and her new fatass? She went through Freshman15.
When a man attempts to have intercourse with 15 college freshman in one night. The guy who completes this great challenge of endurance and skills with the ladies. Is crowned king of the night.