I have always heard the lyric listed on the Internet as "Oh no, I know a dirty word" as "I know I'm not 'the dirty word'". If you listen to the song there is definitely a 't' after the second 'no' sound. When Cobain was young and later, he was very pro-gay rights and hated homophobia, although he was straight. (Watch the doco "Montage of Heck" for proof of this.) In this song, he's attacking people who can't even say the word 'gay' while at the same time saying he's straight. Of course, he did use the dirty word himself eventually, in "All Apologies", and I think this later song is part of the reason he killed himself. For a song that provides a cryptic commentary on Cobain's suicide, listen to "Hey Man, Nice Shot" by Filter. I don't know why the internet is awash with what seems to me a false version of the lyrics. I am sure my sense of the lyrics is correct, that somehow a false transcription gained traction as the official transcription. But then I could be wrong.