What is the origin of the idiom "hot under the collar" and when to use it?

The idiom "hot under the collar" originated in the early 19th century, referring to the physical reaction of the body when experiencing heightened emotions. It symbolizes the visible signs of anger, embarrassment, or irritation, as blood vessels dilate, causing a flushed or red appearance specifically around the neck and face. It is commonly employed in situations where someone becomes heated or agitated due to a specific trigger, such as during arguments, confrontations, or when faced with a humiliating or frustrating event.