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Short for "Get Back In The Halo 3 Forum". Used by members of the Flood whenever someone posts Halo 3 related material in the Flood, which are two forums on bungie.net.
Halo3Idiot: hay guise chek out my stats pls bungie look can i haz recon?

Flood: No, you idiot. GBITH3F.
by Mykester July 25, 2008
"Get Back Into The Halo 3 Forum"

A phrase said when people on the Bungie forums post about Halo 3 in the wrong section.
Guy 1: "Do you like the new maps?"

Guy 2: "GBITH3F!"
by DarkGunner93 July 19, 2008