Up in arms means angrily protesting about something. Residents are up in arms, up in arms over the plan, had them up in arms.
It is used of groups rather than individuals, and it always concerns a decision made by somebody else. The arms are weapons, from an old sense of up meaning in revolt, though nobody is armed now.
1phraseup in armsAngrily protesting about something.
“Parents are up in arms about the plan to close the village school.”
up in arms is a protest.
From an obsolete sense of up meaning in revolt, with arms meaning weapons.
Built from
From a now-obsolete sense of up (“in revolt”).
Etymology from Wiktionary, CC BY-SA
Practised in Idioms