When freshmen in college can't get college girls, so they go back to dating 15 year olds who will put out.
John: Did you see who Kevin was dating now?
Alex: Yeah seriously. Talk about Freshmen 15!
John: You would think a collegekid would stop going after high schoolers.
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 Freshman 15.