not know (any) better

not know (any) better

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!
See also: better, know, not
Farlex Dictionary of Idioms.

not know any ˈbetter

not behave well, politely, etc. because you have never learned how to: You can’t blame him for his bad table manners; he doesn’t know any better.
See also: any, better, know, not
Farlex Partner Idioms Dictionary
See also:
  • do unto others
  • be behind the times
  • bang (people's) heads together
  • bang people's heads together
  • bang your/their/our heads together
  • act out
  • play to the gallery
  • play to the gallery, to
  • be on the same wavelength
  • be on the same wavelength/on different wavelengths