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Angry, rebellious, as in The town was up in arms over the state's plan to allow commercial flights at the air base. This idiom originally referred to an armed rebellion and was so used from the late 1500s. Its figurative use dates from about 1700.
American Heritage Idioms
2
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Phrase(s): *up in arms
1. Fig. in armed rebellion. (*Typically: be ~; get ~.) • The entire population is up in arms. • They are up in arms, ready to fight. 2. Fig. very angry. (Fig. on Q, but without weapons. *Typically: be ~; get ~.) • Wally was up in arms about the bill for the broken window. • I am really up in arms about what happened.
McGrawhill's American Idioms And Phrasal Verbs