one of the most commonly misconstrued sayings, an example used in Boondock Saints "a man is allowed to beat his wife with a stick no thicker than his thumb." A claim made by a satirical caricaturist by the name of James Gillray against a judge. The first and original use of the saying is as simple as the words. The thumb was used as a readily available tool of measuring. Such as the Tailor's rule of thumb "the circumference of the thumb twice, is the circumference of the wrist, twice the circumference of the wrist is the circumference of the neck, and then twice the circumference of the neck is the waist's circumference." Then there is the Brewer's rule of thumb where if the temperature of the wort wasn't insufferable by the thumb it was cool enough to pitch the yeast. It has now been used as a term of commonplace knowledge in a field.
"The rule of thumb in making a sale is confidence, without confidence you don't make a sale"