1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): *down in the dumps
Fig. sad or depressed. (*Typically: be ~; get ~.) • I’ve been down in the dumps for the past few days. • Try to cheer Jane up. She’s down in the dumps for some reason.
McGrawhill's American Idioms And Phrasal Verbs