The story
A reduced form of ordinance, which is attested from the late 14th century in the sense of “military equipment or provisions”. The sense of “artillery” arises in the early 15th century, the sense “military logistics” in the late 15th century. The shortened form ordnance arises by the 17th century, now often distinguished in writing from the other meanings of ordinance. Also doublet of ordonnance.
Etymology from Wiktionary, CC BY-SA 4.0.