up in arms

From a now-obsolete sense of up (“in revolt”).

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up in arms (not comparable)

1. (literal, now uncommon) Armed for battle; prepared for or engaged in warfare.
   • c. 1593 (date written), William Shakespeare, “The Tragedy of Richard the Third: ”, in Mr. William Shakespeares Comedies, Histories, & Tragedies  (First Folio), London: Isaac Iaggard, and Ed Blount, published 1623, →OCLC, , page 199, column 2:

     March on, march on, ſince we are vp in Armes,
     If not to fight with forraine Enemies,
     Yet to beat downe theſe Rebels here at home.

   • 1769, Goldsmith, chapter 18, in The Roman History, from the Foundation of the City of Rome, to the Destruction of the Western Empire. , volume II, London: S. Baker and G. Leigh, ; T Davies, ; and L. Davis, , →OCLC, page 372:

     News was brought him, that the Scythians, and barbarous nations of the North, were again up in arms, and invading the empire with furious impetuosity.

2. (figuratively) Angry; incensed; preparing for a fight.

   The union members are up in arms over threats of pay cuts.

   • 1594, Christopher Marlow, The Troublesome Raigne and Lamentable Death of Edward the Second, King of England: , London: for Henry Bell, , published 1622, →OCLC, [Act I]:

     Lan. My lord, will you take armes against the king?
     Bish. What neede I, God himselfe is vp in armes,
     When violence is offered to the church.

     This usage blends the two senses: the archbishop puns on Lancaster's literal call to arms by invoking God's anger.

   • 1749, Henry Fielding, chapter 5, in The History of Tom Jones, a Foundling, volume III, London: A Millar, , →OCLC, book 18, page 301:

     Allworthy was of a cooler Disposition than the good Woman, whose Spirits were all up in Arms in the Cause of her Friend.

   • 2003 September 28, David Diamond, “The Way We Live Now: 9-28-03: Questions for Linus Torvalds; The Sharer”, in The New York Times Magazine‎:

     Is file-sharing, which has the recording industry so up in arms, the “dark side” of open-source attitudes?

   • 2023 March 24, Nathan J. Robinson, “The Problem With AI Is the Problem With Capitalism”, in Jacobin‎:

     Many artists are up in arms about the new AI programs — and with good reason. Some are furious that their works have been used as training data without their permission.

• "All" may be added as an intensifier at the beginning of the expression, as in:

Her father was all up in arms when we returned late at night.

• in arms
• under arms

• “be up in arms” in Longman Dictionary of Contemporary English, Longman.
• “be up in arms”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.
• “up in arms”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.
• “up in arms” (US) / “up in arms” (UK) in Macmillan English Dictionary.