What is the origin of the idiom "get one's back up" and when to use it?

The exact origin of the phrase "get one's back up" is uncertain, but it likely originates from observing the physical reaction of animals, particularly cats, when they are threatened or agitated. When a cat feels challenged or provoked, it arches its back and raises the fur along its spine as a defensive response. This visual cue has been metaphorically used to describe a similar defensive or irritated reaction in humans. This idiom can be used to describe interpersonal conflicts, debates, or confrontations where emotions run high and individuals react defensively. It can also be used to depict instances where someone's ego or sense of self is at stake, leading them to become defensive or combative.