When bbiab ( Be Back in a bit means leaving for 2 - 5 Minutes ) Doesn't fit.
bbiaw means you will be gone for at least 6 - 10 minutes top.
The reason you would use this is because you have to: use the restroom; Answer the door or phone and would not be able to return in a short time.
Person1:hey did you hear about that thing?
Person1:buzz me when your back.
buzz: To either use YM's Buzz button to get other persons attention or type a letter and press return for same effect.
Although brb means 0 - 15 min, and bbs means 15 - 30 min, and bbl means more than an hour, we lack a term for the 30 to 60 minute range. So, bbiaw is designated for that 30 to 60 minute range, in between bbs and bbl.