To furnish with arms or weapons; to arm or equip for battle.
From 'en-' (causative prefix) + 'arm' (weapon). Middle English formation using the Latin prefix to turn 'arm' into a transitive verb meaning to provide with weapons.
This verb shows how the prefix 'en-' can turn a noun into a verb—'arm' the noun becomes 'enarm' the action of equipping someone with weapons, a pattern you see in 'enable', 'enrage', and 'endanger'.
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