Burning slowly with smoke but no flame, or showing suppressed anger or passion.
From Middle English smolderen, probably from smolder (to suffocate), related to Middle Dutch smeulen. The metaphorical sense of suppressed emotions emerged in the 16th century, paralleling the slow, hidden burn of actual smoldering.
Smoldering fires are actually more dangerous than open flames because they're harder to detect yet can suddenly burst into full combustion. This perfectly captures why we use it metaphorically - smoldering anger or passion suggests something powerful lurking just beneath the surface, ready to ignite.
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