Typical of college students in long-distance relationships, it is the tendency to want to stay in on weekends with one's significant other rather than going out and enjoying time with friends.
Guy 1: Hey, you want to come out with us tonight? We're going to do crazy college things!
Guy 2: No, I have a lot of work to do...
Guy 1: Bull, that's just an excuse to chat with Suzy. Since when did you come down with stick-in-the-mud-itis?