RAB is another of the seemingly endless array of chat
acronyms including
LOL,
ROTFL,
LMAO,
ROTFLMAO,
WTF,
OTOH,
IMHO,
YMMV,
AFAIK, etc.
It stands for
Raises
A Brow.
I believe that it is more common in online
Role Playing comunities than in more general chat
fourms.