The story
From the type of exhaust expelled by the rocket, being composed of π subatomic particles (π mesons, aka pions).
Etymology from Wiktionary, CC BY-SA 4.0.
pion rocket
Today it means: A theoretical type of antimatter rocket which uses proton-antiproton fuel, and magnetic rocket funnels to direct the π-on exhaust. The pi meson particles are created by the matter-antimatter annihilation of protons and antiprotons. Full definition →