Pat Ambrose was laughing as he recalled the origin of Jalen Brunson’s post-up game.

“Rick Brunson used to say he has the Brunson ass,” the high school coach said, referring to the father of the Knicks’ new point guard. “He has this thicker rear end. You don’t want to call it plump or you don’t want to call it big. But, dude, it’s significant.”

It was early in high school when the younger Brunson began to learn how to post up. There were quicker guards. There were faster guards. But nobody back then could handle him in the post. Ambrose, Brunson’s coach at Stevenson High School in Lincolnshire, Ill., remembered opposing coaches telling him how excited his players were to guard Brunson, a five-star prospect. But then they had to defend him in the paint.

“They’d come out of the game, and they’d tell their coach, ‘Holy s–t, is he strong.’ He’d bump them off,” Ambrose recalled. “He would veer guys and kind of knock them off the ball, but he stayed in control. He had this strength, footwork and intelligence that we would capitalize on in high school. It helped isolate him a little bit, and obviously, he’s a great passer.”