To be beside oneself with uncontrollable anger; enraged.
People get upset regularly, and sometimes, they might display fits of anger by yelling or throwing things. Upon seeing this behavior, a person might use the common phrase "foaming at the mouth" to define the person's rage.
This phrase likely originates from diseases or viruses that cause literal foaming at the mouth, e.g. rabies virus. Rabies is a viral disease that both humans and animals can catch. Most cases of human rabies are a result of being bitten from an infected dogs. One symptom of rabies is that it makes swallowing difficult, and for some, the virus can even prevent the person or animal from swallowing at all. Consequently, salvia builds up and there is a "foaming" at the mouth.
This idiom is at least over 400 years old. For instance, in 1601, William Shakespeare, a famous poet and playwriter, used this common expression in the play Julius Caesar:
"He fell down in the market-place, and foamed at mouth, and was speechless."
Note: The phrase origins, or history of particular idioms can be difficult to trace down, so finding the precise time a saying came into existance is no walk in the park. What you will usually find on this site are moments in history where an expression started to be used on a widely-known basis.
For instance, a lot of popular sayings can be spotted in old newspapers from several decades ago, but think about that for a second. If an idiom is being used in the media, it seems apparent that everybody already knows about it! What does that mean? Well, it means the origins of the phrase are probably much older.
knowyourphrase.com - Meanings and Origins of Phrases
* That dude is so angry, just look at him; he's foaming at the mouth and throwing things around, we should probably leave before he catches sight of us.
knowyourphrase.com - Meanings and Origins of Phrases | Interested in sports related phrases? Glance at this sports phrases list then and learn their meanings!