Very angry, indignant, or ready to fight about something; in a state of protest or rebellion.
This phrase originally had a literal military meaning - to be 'up in arms' meant to take up weapons and prepare for battle. The expression dates back to the 1600s when 'arms' referred to weapons rather than body parts. By the 1700s, it had evolved to describe any state of angry protest or outrage.
The phrase perfectly captures how anger can feel like preparing for battle - our bodies actually do prepare for conflict when we're furious, with increased heart rate and adrenaline. It's interesting that we kept the military metaphor even as most people stopped carrying actual weapons, showing how warfare language permeates our descriptions of conflict.
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