Idiotes in classical Greek denotes a private person or layman, contrasted with the citizen active in public affairs and with trained professionals. Thucydides uses the private-citizen sense. Latin idiota carried it toward unskilled and ignorant, Church Latin kept it for the unlettered, and nineteenth-century medicine briefly made idiot a clinical grade before the term was abandoned as a slur.
The favorite retelling claims Athenians fined or despised non-voters and that idiot was their legal label for it. No such law is on record; the contempt was cultural, not statutory. Pericles' funeral speech comes closest, calling the man who minds only his own affairs useless, and that speech is the actual source the story runs on.
Modern commentators, left and right, revive the Athenian sense to scold disengagement: the idiot is whoever opts out of the shared world. The word thus gets drafted into sermons its inventors would recognize.
A language that turned 'minds his own business' into its worst word for a person: what did that civilization value, and what does ours?