1855 in theatrical jargon, "to the rear of the stage," from up (adv.) + stage (n.). From 1901 as an adjective, 1916 as a noun. The notion in the verb (1921) is of drawing attention to oneself (and away from a fellow actor) by moving upstage, so that the other actor must face away from the audience. Related: Upstaged; upstaging.
