down in the mouth is an adjective meaning "filled with melancholy and despondency". It carries one recorded sense.
filled with melancholy and despondency
“gloomy at the thought of what he had to face”
“gloomy predictions”
“a gloomy silence”
“took a grim view of the economy”
“the darkening mood”
“lonely and blue in a strange city”
“depressed by the loss of his job”
“a dispirited and resigned expression on her face”
“downcast after his defeat”
“feeling discouraged and downhearted”
Definitions, synonyms and semantic relations for down in the mouth come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.