That a man was permitted to beat his wife with a switch no thicker than his thumb was a fact, though it was probably not a formal law anywhere nor stated in common law in exactly that form -- and was almost certainly not the origin of the phrase "rule of thumb." Read more about this myth of women's history:

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I never heard that story until very recently and thought it bunk when I did hear it, so I do not associate wife beating with “rule of thumb.” Have no problem using it myself and would not be offended (as a woman) if I heard someone else use it.
More pother in a teacup, if you ask me.