down in the dumps, to be

be down in the dumps

To be visibly sad or depressed. Molly's been moping around all week, and I have no idea why she's so down in the dumps. Rob has been down in the dumps ever since Gloria broke up with him.
See also: down, dumps
Farlex Dictionary of Idioms.

down in the dumps, to be

To be sad or dispirited. The “dumps” referred to are not the modern rubbish heap but a heavy, oppressive mental haze or dullness (from the Dutch words domp and German dumpf). The expression was used several times by Shakespeare (Titus Andronicus 1.1; The Taming of the Shrew 2.1; Much Ado about Nothing 2.3) and was well known as “in the dumps” until the eighteenth century. See also down in the mouth.
See also: down
The Dictionary of Clichés by Christine Ammer
See also:
  • be down in the dumps
  • be down in the mouth
  • come a gutser
  • be down to (one)
  • be down to somebody/something
  • be down to something
  • be down to (do something)
  • bounce up and down
  • close down
  • close down and shut down