1. To not have the knowledge or development to behave correctly or make an informed decision. Don't get so mad at him for writing on the walls—he's two, he doesn't know any better.

2. To not have (contradictory) information about something. Used in the construction "if I didn't know (any) better" to sarcastically or ironically suggest that something is probably or obviously the case. If I didn't know better, I'd be inclined to believe that these megacorporations don't actually care two figs about consumers. Say, if I didn't know any better, I'd think you were actually enjoying yourself, just like I said you would!