“arms”还能表示“武器”。be up in arms这个表达原先指的是“武装叛乱”，后来用来比喻“大动肝火；满腔怒火”。联想一下，生气到都要揭竿而起，拿起武器大打出手了，可见愤怒的程度之深。

【例句】
You might imagine that Americans would be up in arms about all this.
你可能会认为美国人会为此感到满腔怒火。

Mom was really up in arms after I messed up the room.
妈妈看到我把房间搞得乱七八糟，很是愤怒。