The story
From Platonic + solid, in reference to the Greek philosopher Plato, who in his dialogue Timaeus theorised about a correspondence between these solids and the classical physical elements.
Etymology from Wiktionary, CC BY-SA 4.0.
Platonic solid
Today it means: Any of five convex polyhedra with congruent regular polygonal faces, which have a high degree of symmetry and have been studied since antiquity. Full definition →