A British term used to rebuke a small boy who takes favours and presents for granted without giving appropriate thanks: also used after other discourtesies. Usually used by his mother.
Lady Muck is the feminine equivalent.
Mother] Alex, say thank-you when I give you your sandwiches. Who do you think you are: Lord Muck?
Mother] Alice, don't just sit there like Lady Muck when you can see I need a hand with passing round the plates.