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Chiefly as a direction in a score: with full power (specifically in organ playing).
An organ stop or combination of stops producing a full chorus of montres (diapasons and principals) in octaves, plus fourniture (additional mixture pipes, adding fifths), and sometimes cymbal.
Mid 19th century. From French plein jeu organ stop that has a very loud sound, a type of mixture stop combining the fourniture and the cymbal from plein + jeu.