The story
As an idiomatic term, from the 1930s. From the metaphor of filling (priming) a liquid pump in order to prepare it for operation.
First known use, by meaning
- from 1930s To engage in deficit spending.
Etymology from Wiktionary, CC BY-SA 4.0.
prime the pump
Today it means: To engage in deficit spending. Full definition →
First used 1930s