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Related to in arms: up in arms, International Traffic in Arms Regulations
1. Bearing weapons for a fight, as a soldier or military person. With only 40 men in arms, our tiny battalion had little hope of withstanding the siege.
2. In a state of agitated hostility, especially leading to or preparing for conflict. Everyone in the company was in arms after we found out our pensions had been dissolved by the board of directors.
See also: arm