One of the mysteries of the English language finally explained.
Frequently with the. Any of various weapons having a sharpened metal point or blade, as a pike, lance, sword, etc., especially one used by cavalry. Hence in extended use: cavalry; mounted troops. Now historical.
Late 18th century. From French arme blanche from arme weapon + blanche, feminine of blanc white.
arme blanche/ˌɑːm ˈblɒ̃ʃ/
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