Trump says he invented an 84-year-old phrase. But, why?

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7. Long before the invention of centrifugal pumps

The typical vacuum hand pump has been around many hundreds of years. They depend on water above the valve to maintain the seal and thus the vacuum. Most such pumps willl leak away that essential ingredient within minutes or hours, rendering the pump useless for nail that water is replaced. A container of water is usuallly kept by the pump for this purpose. It is poured into the pump, reestablishing the vacuum seal, and priming the pump. "Starter" water. To prime the pump was to get something started.

Has nothing to do with macro economics. A far more general term.

BTW we had several simple hand pumps on our farm including one on the back porch for when the water electricity went out. Once you had primed a pump, the first thing you did was refill the container.