one's back is up

From the way a cat, when angry, raises or arches its back.

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one's back is up

1. (idiomatic) One is offended or angry.
   • 1835, William Hayley, The Life and Letters of William Cowper:

     My back is up, and I cannot hear the thought of wooing him any farther, nor would do it, though he were as pig a gentleman (look you) as Lucifer himself.

   • 2011, Jeannie Watt, The Baby Truce, →ISBN, page 84:

     And I want you to be nice to Patty. For some reason her back is up.

   • 2014, Kristin Butcher, Cabin Girl, →ISBN:

     Now my back is up. “Yes, the witch,” I hiss.

• get one's back up