It's essentially the equivalent of failing to "honour thy father".

There's a glossary of all the phrases and words used in The Dark Tower series:

http://stephenking.com/darktower/glossary.html

> I HAVE FORGOTTEN THE FACE OF MY FATHER
>
> This is a term often used by Roland. It is a phrase of shame. If you have forgotten the face of your father, it means that you have behaved dishonorably.