Here's a breakdown of why it's used:
* Physiological Response: When someone experiences extreme emotions, their body goes into fight-or-flight mode. This can lead to increased salivation and adrenaline, which can cause frothing at the mouth.
* Animalistic Behavior: Foaming at the mouth is often associated with animals, especially wild animals, when they are enraged or threatened. This reinforces the image of intense aggression.
* Figurative Language: The expression is often used metaphorically to describe someone who is incredibly angry, even if they aren't literally foaming at the mouth.
Example:
"The politician was so angry at his opponent that he was practically foaming at the mouth."
Important Note: It's crucial to remember that "foaming at the mouth" can also be a symptom of certain medical conditions, such as rabies, epilepsy, or poisoning. If you witness someone foaming at the mouth, it's important to seek medical attention immediately.