Wiktionary
his back is up
alt. (context idiomatic 1811 English) he is offended or angry; an expression or idea taken from a cat; that animal, when angry, always raising its back. An allusion also sometimes used to jeer a crooked man; phr. (context idiomatic 1811 English) he is offended or angry; an expression or idea taken from a cat; that animal, when angry, always raising its back. An allusion also sometimes used to jeer a crooked man;