@whee is a term found on MUSHes. It's a result of MUSH syntax, which puts an "@" before certain types of commands. It's usually a mildly sarcastic term, but can be used sincerely. Common usage might look something like this:

Starrynight has a Japanese History final tomorrow.  @whee!

There are lots of other spoken @-terms, all of which are generally used when OOC.