Get one’s back up

  Get one’s back up is an idiom that has been in use for some time. An idiom is a commonly used word, group of words, or phrase that has a figurative meaning that is not easily deduced from its literal definition. Often using descriptive imagery or metaphors, common idioms are words and phrases used […]
from Grammarist
https://grammarist.com/idiom/get-ones-back-up/