'Under the weather' means feeling unwell or sick. It does not usually refer to a serious illness. Speakers use it to explain that they do not feel fully healthy without giving medical details.

This idiom is used as a complement after a linking verb such as 'am,' 'is,' or 'feel.' It often appears when someone gives a reason for resting, staying home, or not performing at their best. The tone is informal and suitable for everyday conversation.

Rule
Use 'under the weather' after a linking verb to describe feeling unwell.
Use 'under the weather' to give a reason for reduced energy or absence.
Avoid using 'under the weather' for severe or specific medical conditions.

Speakers often use this idiom to respond to questions about how they are or to explain a change in plans. It can appear with first person or third person subjects. It may be softened with words like 'a bit' or 'slightly' to indicate mild discomfort.

Word/Phrase	Definition
I am under the weather.	A statement that I do not feel well.
She is under the weather.	A statement that she does not feel well.
He is feeling a bit under the weather.	A statement that he does not feel well, with mild emphasis.

English has other informal phrases for minor illness or low energy. These idioms are often used in the same contexts as 'under the weather.' They also avoid naming a specific illness.

Word/Phrase	Definition
Not feeling myself	I do not feel normal or fully healthy.
Out of sorts	I feel unwell or irritable.
A bit off	I feel slightly unwell.

'Under the weather' is an informal way to say that someone feels unwell. It is most common in conversation when explaining minor illness or tiredness without giving medical details.