(Greek, those within). Exoteric, those without. The term originated with Pythagoras, who stood behind a curtain when he gave his lectures. Those who were allowed to attend the lectures, but not to see his face, he called his exoteric disciples; but those who were allowed to enter the veil, his esoteric.
Aristotle adopted the same terms, though he did not lecture behind a curtain. He called those who attended his evening lectures, which were of a popular character, his exoterics: and those who attended his more abstruse morning lectures, his esoterics.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894