An acronym meaning "Your brothers are sleeping together." Originally developed as a temporary replacement, in the Anime-Heaven script of the Naruto 95-96 Special, for a line that no translation had been made of yet.

YBAST was then made into the name of the editing group who worked for Anime Heaven at the time and later composed the majority of the Dattebayo Fan subbing group staff. YBAST is a 5 member (each assigned a letter) fan sub editing team.
There is something I must tell you... your brothers are sleeping together.
-or-
There is something I must tell you...
YHBT

Additional use as a group name:
I saw that YBAST just did another quality Naruto release.
-or-
OMGWTF!NARUTO233 WHY ISNT IT RELEASED YET? MUST BE YBAST'S FAULT!
by Fate March 6, 2005
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