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1859, from French foyer "green room, room for actors when not on stage," literally "fireplace," from Old French foier "furnace, stove, hearth, fireplace" (12c.), from Latin focarium, noun use of neuter of adjective focarius "having to do with the hearth," from focus "hearth, fireplace" (see focus (n.)).
intermediate area between the exterior and interior of a building, especially a theatre. Originally the term was applied only to that area in French theatres, comparable to the greenroom in English theatres, where actors relaxed when they were offstage. Because actors were accustomed to visits by friends during and after performances, such areas came to be large and handsomely decorated.