A sister phrase to the popular "Freshman 15" that refers to how fat Senior girls get due to excessive partying and beer drinking during their final year in college.
Guy #1: That Holly girl used to be hot right, what happened to her? - Guy #2: She let herself go and gained her senior 15 like a fat slob, what a shame.
by Kiva May 11, 2006
Get the Senior 15mug.