BKB spelled out is Baka Kageyama Baka" a phrase Hinata Shoyo uses in Haikyu Season 4 Cour 2 talking about Kageyama
Baka: Moron or Idiot
Literal : Idiot Kageyama Idiot! or Moron Kageyama Moron!
Hinata Shoyo: I thought you were going to toss to me Baka Kageyama Baka ( BKB )!