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Hi Benchong, it’s a  metaphor.  It means the recognized way of doing something.  The exact origin is uncertain.  Some say it started when the thumb was used as a measuring tool.  Many say it came from beer brewers who would use their thumb to check the temperature of the beer.  It is also thought to be an antiquity because it is also used in the Persian language. 

It is often claimed that the term originally referred to a law that limited the maximum thickness of a stick with which it was permissible for a man to beat his wife.  Courtesy of North Carolina Violence Women.

From Gulliver’s Travels by Jonathan Swift:  Then they measured my right Thumb, and desired no more; for by a mathematical Computation, that twice round the Thumb is once around the Wrist, and so on to the Neck and Waist, and by the help of my old Shirt, which I displayed on the Ground before them for a Pattern, they fitted me exactly.  A lot of  info in Wikipedia.  Regards/yvonne57