In Hesiod’s Works and Days, Pandora receives a jar before opening it.
The Greek word used in the story is pithos, a large storage jar commonly used for commodities such as grain, oil, or wine. Hesiod describes the vessel as containing evils that are released when Pandora opens it. The poem does not call it a box.
The “box” wording became established through a Renaissance translation associated with the humanist scholar Desiderius Erasmus. Erasmus translated the Greek vessel as a Latin pyxis, a small box or casket, and that choice influenced later European usage.
Modern images usually show a compact decorated box, but the original vessel could have been a substantial earthenware jar. This translation history explains why the phrase is universally familiar even though the earliest surviving literary source describes a different container.