Gong sun Long (公孙龙), a famous scholar lived in the State of Zhao (赵国) during the Warring States Period (战国,475――221BC), had a habit of maintaining a great number of skilled people around him.
He often said, "A wise man should welcome anyone with a specialty."
One day, a man in shabby clothes came to see him and said to him:" I have a special skill. " Gong asked: "What is it?""I have a loud voice and I'm good at shouting."
Then Gong turned to his followers and asked," Who is good at shouting?" But none of them answered "Yes ". So the scholar took the man in.
Some days later, Gong and his followers went on a trip.
They came to a wide river and found the boat was on the other side of the river. All of them had no idea. Suddenly, Gong thought of the shouting expert and turned to him, "Can you have a try?"
The man realized it was the chance to show his skill. He shouted to the ferryman as loud as he could, "Hey, ferryman, come here, we want to cross the river." As his voice ended, the ferryman came to fetch them. Gong was very satisfied with the new follower.
Later, people use it to describe anyone who has a special professional skill.