One of the mysteries of the English language finally explained.
Military. A body of troops constituting the chief guard, especially a body of cavalry posted on the wings of a camp towards the enemy; a guard in a fortress taking custody of disturbers of the peace, etc.
Mid 17th century; earliest use found in Henry Hexham (fl. 1601–1650), soldier and author. From main + guard.
main guard/ˈmeɪn ˌɡɑːd/
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