Rule of thumb

 

When we use established past practices as a guideline for making a decision, we often refer to "rule of thumb." 

In 1732 Francis Buller, an English judge, proclaimed that a "man could not beat his wife with a stick larger than the diameter of his thumb." Regardless of Buller's intention, his "rule of thumb" was taken seriously by many, resulting in a large public outcry accompanied by satirical cartoons.  The remark was never forgotten, as it was attributed to him in biographies written after his death.

While Buller is credited with the phrase's origin, in reality it was probably used much earlier: The "thumb" was a unit of measurement in the late seventeenth century.