Extremely angry or furious, with anger so intense it seems to generate heat like steam. The phrase suggests visible signs of rage that others can observe.
This expression extends the metaphor of anger as heat, specifically referencing steam as the visible manifestation of boiling water. The phrase became popular in the 19th century when steam engines were common, making steam a familiar symbol of contained pressure seeking release.
What makes 'steaming mad' particularly vivid is that steam is both invisible (water vapor) and visible (condensed droplets), much like how anger can be both an internal feeling and an external display. The phrase also suggests that the anger is so hot it's literally evaporating, creating a visible cloud of fury that others can see from a distance.
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