Someone who has been involved in a
scene (such as
MUDding,
IRC,
Everything2, and so on) for a good percentage of the amount of time that scene has been
around. This
value, for me, is usually 50%. Oldbies are usually very
experienced, due to the sheer amount of
time they spend in the
scene. They sometimes create related scenes of their own.
There is also something which I like to call oldbie syndrome. This doesn't happen to all oldbies, but it definitely happens. Oldbies sometimes become withdrawn and don't associate much with the "younger" crowd. They sometimes have incredible knowledge of the scene, but refuse to help newbies, or in extreme cases, outright dislike them. People with oldbie syndrome also tend to hang out mostly in cliques with other oldbies, and rarely allow others (even other oldbies, in some cases) into their clique.
One reason that I can attribute to this behavior is familiarity. When you first join something new, for example, a TinyMUCK, it is a new world to explore, with new people and new commands to learn. It becomes more or less an adventure, trying to figure it all out and make new friends in the process. But, as the years go by, you become more familiar with it. You become less excited by the scene, and only really stick around to stay in contact with the friends you've made. You tend to reminisce about the good old days and wish they'd return; what you don't realize is the reason the "good old days" were so good is because you were a newbie and exploring a new world was lots of fun.
An oldbie can instantly become a newbie when entering a similar but different enough to be confusing scene. For instance, someone who's been on TinyMUCKs for many years will find himself behaving just like a newbie on a MUSH. Nothing is more amusing than an oldbie totally confused by a system he is not familiar with. }:)