What do we mean when we say it today?
Most people aren't grabbing for their guns when they are "up in arms" today. We don't take it so literally. Typically it's used when someone is getting all hot and bothered. We'll go ahead and say it. Most of the time, that means the person shouldn't necessarily get so angry. "Up in arms" often describes someone who is overreacting to something. They get all up in arms, or furious, when they shouldn't.