I'm an AP Psychology student, and my teacher said that the phrase rule of thumb originated from an (obviously) outdated English law that stated that it was acceptable to beat your wife, but only if the stick was less than the diameter of the thumb. Stunned, I went to all my usual etymology sources to find out the truth. However, seven out of eight sources concurred that this is a myth. While rule of thumb, meaning "a broadly accepted guideline", a phrase combining the descendant of Latin for "straight stick" (a future etymology post) and the offspring of the PIE word meaning "to swell", was an actual thing in Great Britain, it was neither a law nor the origin of the word. Of the five sources that explained where rule of thumb actually came from, they all agree that it was established more than a century before the English incident, as a reference in woodworking, where the rule of thumb was literally that the first joint of a thumb was about an inch long. Just wanted to dispel that misconception...
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd