What world does he inhabit? The general consensus is that the phrase dates from the 19th century.
From the transcript of Trump's interview with The Economist. Trump's responses are in bold.
But beyond that it’s OK if the tax plan increases the deficit?
It is OK, because it won’t increase it for long. You may have two years where you’ll…you understand the expression “prime the pump”?
We have to prime the pump.
It’s very Keynesian.
We’re the highest-taxed nation in the world. Have you heard that expression before, for this particular type of an event?
Priming the pump?
Yeah, have you heard it?
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.
Yeah, what you have to do is you have to put something in before you can get something out.