The story
From the initial letters of the names of the eight planets, plus Pluto, formerly considered a planet.
Etymology from Wiktionary, CC BY-SA 4.0.
my very educated mother just served us nine pumpkins
Today it means: Used to remember the order and names of the (then) nine planets of the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. Full definition →