Literally, the inkdrinker-coined "BRB FBI" stands for "be right back; federal bureau of investigation." The implication is that something one has just mentioned is illegal and that the FBI are at the door. Many variants of it are popular at YTMND
"So how old is your girlfriend anyway?"
"Just 15. Oh shi - BRB FBI."
be right back, (the) FBI (is at my door)
mark> Isn't that illegal?
BRBFBI is a meme that originated from srs hackers who were busted during a srs LAN party. They all were killed by a rock slide. However, their phrase of choice lives on. It is the phrase of choice that you say when the feds do elite IP tracing and come to arrest you for downloading kiddie pr0n
and music off the intertubes. They then interrogate you and take you to the pokey where you get loved.
This phrase is frequently heard during the ensuing lulz
and drama of an alleged chan raid.
When uttering the phrase, all caps is necessary. Otherwise, the lulz of the phrase is lost, and you must get raped. A less funny version, "BRBFEDS," may be used as well, however the true inner meaning might be lost.
Person one: ohaider
Person two: I haxed yer daterbase
Person one: orly?
Person two: BRBFBI