Literally: mind – spirit – extinguish – sink
Alternately: Discouraged. Disheartened. Despondent. Not sad-sad or depressed per se, but feeling down and lacking energy, as after a setback.
Notes: 意気 is one’s spirits or sense of enthusiasm; we’ve actually seen it before. (And there are other phrases that include it as well, many of them positive.) 消沈 is when something, especially one’s energy, disappears.
The 消 character may also be written as equivalent characters 銷 or 悄.