Case 10: Bokushu's "Idiot"

Bokushu asked a monk, "Where have you come from?"
At once the monk shouted, "Kaatz!"
Bokushu said, "The old monk has been scolded by you with a 'Kaatz'!"
The monk shouted again, "Kaatz!"
Bokushu said, "After three or four shouts of 'Kaatz', then what?"
The monk was silent. 
Bokushu hit him saying, "You idiot!"