Donald Trump confounded The Economist when he claimed during an interview with the magazine that he came up with the phrase "priming the pump" — a popular economic metaphor that predates his birth.

The US President was discussing how he's OK with his plan to reduce taxes and increase the deficit in the short term, because it would increase revenue over time.

This was the exchange:

Mr Trump: We have to prime the pump. The Economist: It's very Keynesian. Mr Trump: We're the highest-taxed nation in the world. Have you heard that expression before, for this particular type of an event? The Economist: Priming the pump? Mr Trump: Yeah, have you heard it? The Economist: Yes. Mr 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. The Economist: It's... Mr Trump: Yeah, what you have to do is you have to put something in before you can get something out.

Eyebrows were raised at Mr Trump's suggestion that he came up with the phrase a couple of days ago because

It's an old and common phrase

It's an old and common phrase It's probably not the sort of metaphor you'd come up with in modern society

Oh, and Mr Trump himself has used the phrase before. "Sometimes you have to prime the pump," he told Time magazine in an interview last year.

So when was the phrase priming the pump first used?

The Merriam-Webster Dictionary has given us all a helpful history lesson on Twitter:

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In its economic sense, the phrase originated during US president Herbert Hoover's response to the Great Depression. A book titled Pump-priming Theory of Government Spending was released in 1939.

This was all before Mr Trump was born in 1946.

What does it mean?

It generally refers to the government getting the economy moving through government spending, but it can also apply to other stimulants like tax cuts or lower interest rates.

As an example, the phrase was used to describe former prime minister Kevin Rudd's stimulus package during the global financial crisis.

"There's only so much pump priming any government can do," the ABC's Michael Brissenden reported on The 7.30 Report in 2008.

But how does it mean that?

You'd be forgiven for being confused. Because unless you have a well that you use to pump for water, you probably can't picture the metaphor.

In its original sense, "priming the pump" means pouring water into a pump in order to get it working. It's complicated, but basically, there needs to be some water already in the system before it can work.

So in the economic sense, it means pouring money into an economy in the hope that it gets things going.

The phrase is often associated with the theories of influential economist John Keynes (1883-1946), who argued governments can play a role in stimulating economic activity.

Could Mr Trump have been joking? Could he have meant something else entirely?

Yes. Mr Trump wasn't asked to clarify what he meant, so we may never know.