Answer:
The phrase, "rule of thumb" refers to a rule or guideline that is usually though not always correct, it lacks strict precision.
Its origin is very simple; it is based on the use of one's thumb as a measuring tool that, of course, can only give approximate measurements. Most Old English measurements originated from the bodily dimensions of the king such as the length of the "foot" for one foot, the inch (thumb tip to first knuckle), cubit (elbow-to-fingertip), and yard (nose-to-fingertip).
There is a second origin given for this phrase. In early English law, and potentially brought over in colonial American, a man was allowed to discipline his wife by beating her provided he used a stick no greater than the diameter of his thumb
Rule of thumb was a term, a miller would use when grinding flour.
He would rub the flour between his thumb and forefinger, if it was too course he would lower the grinding stones to make the flour finer.