What does the phrase 'Rule of Thumb' mean? Find out the phrase's definition & origin, and get examples of how to use it in a sentence. What is a 'Rule Of Thumb' A rule of thumb is a guideline that provides simplified advice regarding a particular subject. It is a general principle that gives practical instructions for accomplishing or approaching a certain task. Typically, rules of thumb develop as a result of practice and. Definition of rule of thumb in the Idioms Dictionary. rule of thumb phrase. What does rule of thumb Alternatives to business valuation rules of thumb for.