Answer:
This is an urban legend. There was no such law. The phrase "rule of thumb" comes, rather, from the time-honored practice of using the thumb to make rough measurements (think "rule" as in "ruler"). In some languages, in fact, the word for "inch" is the same as the word for "thumb" (e.g., French pouce) or is derived from it (e.g. Spanish plugada, from pulgar).