According to urban legend, the "rule of thumb" originated from a law in the 1400's stating that a husband could beat his wife as long as the stick was no thicker than his thumb.
The phrase dates back at least to 1692, when Sir W. Hope, Fencing-Master, wrote "What he doth, he doth by rule of Thumb, and not by Art."
In the 19th century there were laws in England regarding wife-beating and thumb-thick rods.[1,2,3,4] However, there is no written record connecting the laws to the phrase any earlier than the 20th century, and the link has been labeled a 'feminist fiction' by researchers.
Not only that, but as Shaeffer notes, while abolishing it altogether would have been better, the original laws were actually improving the situation for women at the time by restricting the type of weapon husbands could use.
Where I learned it:
An email of 'Interesting facts' from my brother Andrew.