Definition & Meaning of "{one's} blood [is] up"

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

While the exact origin of the idiom "one's blood is up" is challenging to pinpoint with precision, it can be traced back to at least the 16th century in English literature. The phrase likely emerged from the belief in the Humoral theory, which associated different temperaments and behaviors with the balance or imbalance of bodily fluids, including blood. The phrase is often used to depict individuals who are visibly agitated, passionate, or ready to confront a challenge. It can be applied to various scenarios, such as heated arguments, intense competitions, or moments of deep personal conviction.