Feeling ill, unwell, or slightly sick. Not in good physical condition or health.
This phrase comes from 19th-century nautical terminology. When sailors felt seasick, they would go below deck to get away from the harsh weather conditions that were making them feel worse. Being 'under the weather' literally meant being sheltered below deck due to illness.
The maritime origin explains why this phrase specifically suggests mild illness rather than serious disease - seasickness was uncomfortable but temporary, just like most cases where we use this expression. It's also a rare example of a nautical phrase that kept its gentle, understated tone rather than becoming more dramatic on land.
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