Lol. It's you who don't understand. He claimed to have invented the phrase.
TRUMP: We have to prime the pump.
ECONOMIST: It's very Keynesian.
TRUMP: We're the highest-taxed nation in the world. Have you heard that expression before, for this particular type of an event?
ECONOMIST: Priming the pump?
TRUMP: Yeah, have you heard it?
ECONOMIST: Yes.
TRUMP: Have you heard that expression used before? Because I haven't heard it. I mean, I just...I came up with it a couple of days ago and I thought it was good. It's what you have to do.
Edit: That was a pathetically weak deflection. Do better. Also note that he tried to pull that after the interviewer said he had already heard it.