"Who is that girl? I've never seen her before."
"That's Nancy, she lives across the hall. She's a total dorm rat."
A girl in a dorm setting that "runs through" or sleeps with just about every dude in that paticular dorm...possibly other dorms as well.
New guy: "look at her! who is she?"
You:"dude dont talk to her you might catch something shes a dorm rat."
New guy: "really thats a shame she looked good to"