The English idiom "foam at the mouth" is easy to understand once it has been explained. It simply means to be extremely enraged. The expression comes from one of the symptoms of rabies seen in dogs, cats, and other mammals. A person who begins to foam at the mouth is dangerously out of control with anger, cannot be calmed or controlled, and should be avoided.
Rabies is less common now than it once was, so the expression isn’t as transparent as it would have been when it was first coined. In the earlier part of the 20th century, even urban dwellers had more opportunity to see animals that were rabid because, even after vaccines became available, many pet owners didn’t vaccinate. In addition, pet dogs and cats were permitted free access to the streets and mingled there with feral animals. It was not uncommon for animals in homes as well as in the streets to suddenly begin to foam at the mouth and display other signs of rabies.
Rabid animals lose all control and appear to be crazed with anger. They are unable to swallow, so bubbly drool falls out of their open mouths. Rabid animals will attack anyone who comes near, even a beloved owner or someone who is trying to help.
The expression is a form of hyperbole. Hyperbole is a literary device that exaggerates or overstates an idea or image for effect, and it is common in idioms such as this one. Obviously, even the most irate people don’t literally foam at the mouth the way a rabid animal does. Nonetheless, when a boss, spouse, or even a stranger is so furious that all control has been lost, the target of the anger might feel as afraid as they would if that person were indeed rabid.
Not only used to describe someone else, people often incorporate this idiom into a story they are telling about how something made them very angry. The typical narrative in which the speaker self-identifies as foaming at the mouth is generally delivered with a sense of amazement, as though the speaker’s behavior was so far outside the norm that it was unrecognizable. This aligns with the reality of rabies because rabid animals behave against their natures. Nocturnal creatures such as raccoons or bats appear during the day, passive pets become vicious, and energetic animals behave as though they are exhausted.
Rabies is transmitted through bites, and people in the presence of a rabid animal should move away from it as quickly as possible. Anyone who is confronted by someone “foaming at the mouth” is also best off stepping away from the situation. That is because those who have reached that level of anger are, like truly rabid animals, far beyond reason.