To become visibly sick, nauseous, or envious, with one's face taking on a greenish pallor. Most commonly used to describe the physical appearance of someone feeling ill or extremely jealous.
The association between green and sickness comes from the observation that people who are nauseous often develop a pale, sickly complexion that appears greenish. The connection to envy ('green with envy') dates back to Shakespeare and ancient Greek literature, where green was associated with jealousy and bile.
It's remarkable how green became associated with both physical illness and the 'sickness' of envy - suggesting that jealousy was seen as literally unhealthy. The phrase also demonstrates how color metaphors in language often reflect real physiological changes, as people who are nauseous do sometimes appear to have a greenish tint due to changes in blood circulation.
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