Green’s Dictionary of Slang

(US campus) a student living in (and rarely leaving) a college dormitory.

1963		F. Kohner Affairs of Gidget 32: I wish you’d promise me not to spread the word among your dormies.
1980		Eble Campus Sl. Fall 5: rat – person known to reside in certain decrepid places, mainly dorms and frat houses: ‘How long have you been a dorm rat?’.
1989		P. Munro Sl. U. 72: dorm rat/dorm frog a student who lives in the dorms for more than a year or who rarely leaves the dorms.