Wait till he finishes talking before spraying him.
"喷", adj. For describing a violent spray of liquid.
It is often used as a verb on Chinese internet; an act of talking offensively toward a certain subject, driven by sheer disapproval.
This is a memetic warning usually displayed in the middle of a mass conversation. It can be a mild way of warning someone that he is getting over the edge, or a light-hearted disapproval for something far less serious.