Yabaku is a fairly enigmatic tribe out of ten prominent Türkic tribes enumerated by Mahmut Kashgari (11th century) in the list describing the location of the Türkic polities from the borders of Greece to the borders of China in the following sequence: Bechen (Badjinak), Kyfchak (Kipchak), Oguz, Yemek (Kimek), Bashgyrt (Bashkort), Basmyl, Kai, Yabaku, Tatars, Kyrgyz. Mahmut Kashgari noted that the tribes Kai, Yabaku, Tatar, and Basmyl are bilingial, speaking in Türkic alongside their own languages, while Kyrgyzes, Kyfchaks, Oguzes... have their own Türkic languages, and the languages of Yemeks and Bashkirts are close to them.
References
Yabaku Wikipedia(Text) CC BY-SA