That expression doesn’t apply to people, only things, like events in your life. So it’s always in the third person. You can say „Life’s a bummer“ or „that’s a bummer“ but people can’t be bummers. And like Luiz said, it expresses disappointment or dissatisfaction with something. „Oh, you got the flu? Man, that‘s a bummer.“. Plus it’s very colloquial and it’s never used for really tragic things like diseasters in life. You’d never say, „Grandmother is dying of cancer.“ oh, that’s a bummer.“ or „10,000 people died in that earthquake. „ Oh, that‘s a bummer.“ But, „grandmother got a cold last week.“ „oh, that’s a bummer“, or „The lady got a bruise on her head from that minor car accident.“ „Oh, that’s a bummer.“. Big difference, so be careful how you use the word. I’m a speaker of US English, so I don’t know about its use in other countries. The word „bum“ exists here and that’s the vagabond on the street. That’s obviously different from the UK use of the word. But here, bummers are only things and they’re only annoyances, never anything serious. You’d never say a nuclear war „would be a bummer“. To say that would grossly trivialize to possibility to the point of the absurd.