Attributive use meaning ‘disappointing’ or ‘unpleasant’. Treated here as an adjective in line with guideline for attributive nouns.
Definitionen
Attributive, informal: relating to or characterized by disappointment or unpleasantness. It was a real bummer day when the campsite flooded.
Used informally as a reaction expressing disappointment or sympathy.
Definitionen
Used to express disappointment, frustration, or sympathy. "You lost your keys again? Bummer!"