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mid-15c., from Middle French municion "fortification, defense, defensive wall" (14c.), from Latin munitionem (nominative munitio) "a defending, fortification, protecting," noun of action from past participle stem of munire "to fortify," from moenia "defensive walls," related to murus "wall" (see mural). By 1530s the sense had passed through "military stores" to become "ammunition."