You daft boy. I think this was my dad's favourite rebuke, certainly his mildest. He would also use "Yer daft 'ap'orth". I find myself using the former more and more with my own boys. To call someone daft is usually a mild complaint. Like the word silly its etymology suggests a movement from a positive to a negative meaning. Silly once meant holy and daft once meant meek. It is not difficult to see how such shifts take place.