(one's) back is up(redirected from back is up)
(one's) back is up
One is angry, hostile, defensive, or irritable, often due 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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.