What was Big Willy Shakes going for?
You don't want to mess with someone who's up in arms. But we're betting you knew that already. That person is enraged, provoked, ticked off…you get the idea. The Captain is saying that Warwick shouldn't be angry and fighting against the king. He should put down his weapons and live peacefully.
Um, okay. What do arms have to do with anything? This whole thing makes way more sense when we tell you that arms are weapons. Think about the word armor or the right to bear arms. People aren't talking about hands, elbows, and shoulders. They're talking about weapons. If we think about what that means in the phrase, and replace the word arms, then it looks like this: up in weapons.
Makes a lot more sense now, doesn't it? So the Captain is telling Warwick that he shouldn't get mad and use weapons; he should instead get over himself and play nicely with others. If only he learned that lesson in kindergarten like everyone else.
Shakespeare came up with this angry little phrase first in this play, but his arms, er, hands used it again in Richard III and Titus Andronicus.