freshman 15

15 is said to be the number of pounds that students gain in the first college year

and freshman 15 is the observed phenomenon of  weight gain among first year college students.

据说，之所以会出现15这个数字是因为很多学生在大一这一年体重会增加15磅；而freshman 15则指代学生上大一时体重增加这个现象