(one's) back is up

(one's) back is up

One is angry, hostile, defensive, or irritable, often owing to a particular cause. John's back is up now. You shouldn't have brought up the subject of college. It's election season, so my dad's back is up all the time. No good will come of calling them while your back is up—wait till you're calmer.
See also: back, up
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.
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