The expression "prime the pump" has been used in economics since the Great Depression.

But President Trump claims he coined it the other day.

He bought it up in an interview with The Economist after he was asked whether his tax reform proposal might increase the federal deficit.

"We have to prime the pump," he said, meaning stimulate the economy.

"Have you heard that expression used before?," he continued. "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."

But the term is most often associated with 20th century economist John Maynard Keynes, a giant of the field and a favorite of liberals who favored government spending.

Keynes, generally thought of as the intellectual father of the New Deal, wrote and spoke of the need for government to spend money on projects during times of economic weakness to get the economy moving again.

Trump is also an advocate of spending on public works like highways and airports. But in this case he was advocating tax cuts to spur spending, and thus "prime the pump" to greater economic growth.

Related: Trump: I might release tax returns -- when I'm out of office

Merriam-Webster traces the phrase itself to Sir Walter Scott in 1819, though he wasn't talking about economics. The dictionary company, which has made a habit of fact-checking the president, corrected him Thursday.

The phrase 'priming the pump' dates to the early 19th century. — Merriam-Webster (@MerriamWebster) May 11, 2017

The economic sense dates to a Wall Street Journal article in 1933, according to Merriam-Webster. President Franklin D. Roosevelt used it in a 1937 speech.

Keynes did not actually use the term "prime the pump" or "pump priming" in his seminal work, "The General Theory of Employment, Interest and Money" in 1936, but the term is widely associated with him nonetheless. Google lists more than 21,000 entries that include both his name and "prime the pump."

Related: Donald Trump thinks he invented the phrase 'priming the pump.' That's telling.

"Certainly, Keynes is associated with the idea, but it doesn't mean he used that exact phrase," said Peter Sokolowski, editor at large at Merriam-Webster.

For that matter, the phrase didn't first occur to Trump a couple of days ago.

"Sometimes you have to prime the pump," he told Time magazine in December, speaking of infrastructure spending and tax cuts. And he used the phrase three times in an interview with The New York Times Magazine in March, even stopping to make sure his interviewer got the drift.

"You know what I mean by 'prime the pump'?" he said. "In order to get this going, and going big league, and having the jobs coming in and the taxes that will be cut very substantially and the regulations that'll be going, we're going to have to prime the pump to some extent. In other words: Spend money to make a lot more money in the future. And that'll happen."