He's about the one sort of character nobody knows how to deal with, for he sets everybody else but himself down as duffers.

What can anybody do to such a one?
But there is another extreme.

Billy's great fault was that he was too ready to believe others who called him a duffer.

Don't take it for granted you are a duffer because any one tells you so.

Find it out for yourself, and when you've found it out--"don't be a duffer!".