To draw attention away from someone else; in theater, toward the back of the stage away from the audience.
From theater terminology combining 'up' + 'stage.' Originally a literal directional term, it gained the figurative meaning of stealing someone's spotlight in the early 20th century.
Theater actors learned that moving upstage (toward the back) forces the audience to look there instead of at the downstage actor—so the term became synonymous with deliberately stealing someone's attention or moment.
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