[Burner]

Do any historians out there know the origins of the old rule of thumb?

Many moons ago an English Judge ruled it lawful for a man to beat his wife with a stick provided the stick was no thicker than his thumb.
Hence, the "Rule of Thumb".

Latter day use of the term is related to approximate measurements
or commonly held beliefs.

"As a rule of thumb" one would hit a low running shot from uder overhanging tree branches, for example.