The difference between Off one's feed and Under the weather
Off one's feed is also phrase with the meaning: not eating normally.
Under the weather is also adjective with the meaning: somewhat ill or gloomy.
check bellow for the other definitions of Off one's feed and Under the weather
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Under the weather as an adjective (idiomatic):
Somewhat ill or gloomy.
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Under the weather as an adjective (idiomatic):
Somewhat intoxicated or suffering from a hangover.
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Under the weather as an adjective (idiomatic, obsolete):
Experiencing adversity.