In the Warring States Period, King Xuan of the State ofQiloved to listen to the yu — an ancient wind instrument. He would order 300 musicians at a time to play the yu for him. Mr Nan Guo, who couldn’t play the instrument, passed himself off as one of the musicians. When King Xuan died, his son King Min succeeded to the throne. King Min also loved yu, but he preferred solo performances. Mr Nan Guo thereupon slipped away from the orchestra.
This idiom is used to describe those who have no actual skills but pretend to be experts, or the passing off of inferior things as high quality ones.