down in the mouth

Also, down in the mouth. Discouraged, depressed, or sad, as in She's been down in the dumps ever since she lost the match, or What's wrong with him? He's so down in the mouth about everything. The noun dumps has been used for "a state of depression" since the early 1500s, and down in the mouth, alluding to the downturned corners of the mouth as a sign of misery, dates from the mid-1600s.

The adjective has one meaning:

Meaning #1: low in spirits
  Synonyms: blue, depressed, dispirited, down, downcast, downhearted, low, low-spirited