Feeling sad, disappointed, or let down about something that didn't go as hoped.
From 'bum,' originally meaning a vagrant or person of low status (from possibly German 'Bummler'), the verb 'to bum' evolved in American slang to mean feeling dejected.
The word 'bum' has completely different meanings depending on context—in American English it means sad, in British English it means buttocks, and in Australian English it means vagrant—one of those funny false cognates between English dialects!
Complete word intelligence in one call. Free tier — 50 lookups/day.