تعریف و معنای "down in the mouth"

What is the origin of the idiom "down in the mouth" and when to use it?

The phrase "down in the mouth" originated in the late 18th or early 19th century, describing someone who appears sad or dejected based on the physical expression of a drooping mouth. It can be used to describe someone feeling low after a setback, someone expressing their dissatisfaction or frustration, or someone who is generally feeling downcast.