Feeling under the weather can be naturally or purposely (self) inflicted. When it’s naturally inflicted, one can come down with a head cold because of cold temperatures. When it’s self-inflicted, it is because one is feeling intoxicated or has a hangover. To feel under the weather can also mean to feel upset. This idiom is occasionally heard in a professional environment, like when someone is sick and can’t attend work. It is more often heard in social situations.
Further Examples:
The little boy couldn’t go out to play because he was feeling a little under the weather.
The manager postponed the presentation as he was feeling under the weather.
