armour: [13] Armour comes ultimately from Latin armātūra ‘armour, equipment’, a derivative of the verb armāre ‘arm’ (the direct English borrowing armature [15] originally meant ‘armour’ or ‘weapons’, but the ‘protective’ notion of armour led to its application in the 18th century to ‘metal covering the poles of a magnet’). In Old French armātūra became armeure, and subsequently armure, the form in which it was borrowed into English (the -our ending was artificially grafted on in the 14th century on the model of other Latin-based words such as colour and odour). Armoury is French in origin: Old French armoier ‘coat of arms’ was a derivative of arme ‘weapon’; this became armoirie, which was borrowed into English in the 15th century as armory, meaning ‘heraldry’, but also, owing to their formal similarity, came to be used with the same sense as armour – ‘protective metal suit’ or ‘weapons’.
This was what armoury meant when it came into English in the 14th century (and the sense survived long enough to be used by Wordsworth in a sonnet to ‘Liberty’ 1802: ‘In our halls is hung armoury of invincible knights of old’). The meaning ‘place for keeping weapons’ developed in the 16th century. => armature
