Producing lots of bubbles or foam on the surface, or extremely angry.
From Old English 'fam' (foam), related to Germanic roots. The word combined with '-ing' to show the action or state of producing foam. The 'angry' meaning came from observing animals that foam at the mouth when enraged.
Shakespeare used 'foaming at the mouth' to show rage, connecting human anger to rabid animals—a vivid metaphor that's lasted 400 years because our brains actually link fury with loss of control over our mouths!
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