hen someone is said to be “down in the mouth,” it means that he or she seems depressed or dispirited. Similar terms include “in the dumps,” “feeling the blues,” or “bummed out.” Many cultures have some form of expression for describing someone who is depressed, using the turn of phrase as shorthand to describe someone's state of mind.
English speakers have been using “the dumps” to refer to a depressed state since the 1500s, and the expression “down in the mouth” emerged around the mid-1600s. It's a reference to the turned down corners of a frown, as most people associate frowns with upset or unhappiness. When someone is described in this way, it literally means that his or her mouth is composed in a downturned expression, because of a general state of glumness that precludes smiling.