Quoting Thorian's Mommy:" Can a five month old really understand the word no? i really dont want deny him my boobs cuz we just went through a strike and i would hate for him to think i dont want to bf?"
If you say it in a voice that isn't playing, kind of. With DS2, I would just pull him away and tell him "no, we don't do that" without smiling or anything (I wasn't being mean either) and then gave him the boob back. If he bit down on it again, I would do it again.
But if you make a loud noise and react to it (which I know is hard not to do) then he will start thinking it's a game or it's funny.