Abbreviation used all the time by Millenials
An abbreviation of 'be right back'. It was most often used online in various messaging platforms, but is now often spoken aloud in-person when needing to temporarily leave a group or a room within the need of explanation (such as needing to use the toilet).
"Brb." ... "Okay, I'm back." "Welcome back."
11 out of 11 English speakers have heard this