What is the origin of the idiom "under the weather" and when to use it?

The idiom "under the weather" has origins in the sailing world and dates back to the 18th century. It originated from sailors who would often feel sick or unwell due to rough weather conditions, which could cause them to feel unwell or seasick. The phrase evolved to describe someone who is feeling sick or not in good health. It is used to indicate that someone is feeling unwell or experiencing a mild illness. It suggests a temporary state of physical discomfort or sickness.