Dumb is a song on Nirvana's third album, In Utero, released in 1993. Although this was Nirvana's third album, it was their second major label album, after the breakout Nevermind.
When Nevermind was recorded and released, Kurt Cobain was as anonymous as a minor label musician in Seattle could be. It wasn't expected that anyone would search his lyrics for biography, even if anyone could understand him. After Nevermind became popular and Kurt became one of the rock stars that he despised, his personal life became tabloid fodder. A large part of that was his addiction to heroin, a substance that he apparently had not yet used when recording Nevermind.
"Dumb" may or may not be a heroin song. Unlike Lou Reed (amongst many other rock stars), Kurt Cobain never seemed to want to become a drug addiction celebrity, of either the glamorous or dirty variety. Although his songwriting was always personal, he didn't seem to have the desire to write explicit biography. And as far as heroin went, he probably wanted to avoid glamorizing it for his mostly teenage fanbase.
"I'm not like them
But I can pretend
The sun is gone
But I have a light
The day is done
But I'm having fun
I think I'm done
Or maybe just happy"
Sung, like many of the songs on In Utero, in a soft singsong with minimal musical backing, it has a droning, lulling quality. I don't know from listening to this whether it is a song about drugs, or a song about depression, or maybe just a song about the long Seattle winters. Whatever the subject matter, it showed that Cobain could be a wonderfully evocative songwriter, even if it wasn't clear what he was evoking.