Definition of know better
1 : to be smart or sensible enough not to do something You walked home alone? Don't you know better (than that)? She'll know better than to trust them again. There's no excuse for his behavior. He's old enough to know better. Don't blame him. He's just a child and he doesn't know (any) better.
2 : to know or understand the truth about something She tried to tell me that it wasn't her fault, but I know better.
3 : to know or understand more than other people You can't tell him what to do. He always thinks that he knows better.