ok Here is the smart persons description

ok i researched this a bit and here is what I found: These are both two different excerpts from people that I found online.

The term "rule of thumb" has been used to mean "rough or approximate measure"
for several hundred years. This is amply documented in the Oxford English
Dictionary and various other works. On the other hand, there are apparently
legal cases in both the United States and Great Britain in which judges estab-
lished a standard for the maximum diamater of a rod with which a man may beat
his wife.I use the word "apparently" because I have not actually gone to the
law library to retrieve these cases. What is clear is that the term "rule of
thumb" does not have its origin in rulings regarding violence toward women; it
predates any legal opinions on the issue.
I
think it was originally an expression of measurement. In fact, there
is a word in Swedish, "tum" (=inch), which originates from the word
"tumme" (=thumb). So -- I would strongly suspect that "the rule of
thumb" originally was used by carpenters and others as a measuring
device.