What is the origin of the idiom "cool as a cucumber" and when to use it?

The exact origin of the idiom "cool as a cucumber" is uncertain, but it has been in use since the late 17th century. It likely arose from the association of cucumbers with a cool temperature and the idea of maintaining composure. The phrase is used to describe someone who stays calm and collected in difficult situations. It is used in various contexts to describe individuals who remain calm and composed in challenging or stressful situations. It can be applied in personal conversations, discussions about someone's demeanor, or even in professional settings.