In the Post's review of Wendy Moore Crown's biography of the Countess of Strathmore, Araminta Wordsworth writes:
In telling the countess's story, the author has done nothing less than illuminated the punitive rules that underpinned Georgian society. It is shocking to realize that the elegant women who look out of Gainsborough or Reynolds' paintings became their husbands' chattels on marriage and could legally be beaten by their spouses. The only proviso, a celebrated jurist mandated, was that the stick be no thicker than a man's thumb.
Shocking indeed, especially since the "rule of thumb" anecdote is a complete crock, and the celebrated jurist referred to said no such thing. I'll leave it to Hoff Summers to explain:
As it is told in the opening essay in one of the most popular textbooks in women's studies, Women: A Feminist Perspective, "The popular expression 'rule of thumb' originated from English common law, which allowed a husband to beat his wife with a whip or stick no bigger in diameter than his thumb. The husband's prerogative was incorporated into American law. Several states had statutes that essentially allowed a man to beat his wife without interference from the courts."
The story is supposed to bring home to students the realisation that they have been born into a system that tolerates violence against women. Sheila Kuehl, the feminist legal activist who had played a central role in launching the "Abuse Bowl" hoax, appeared on CNN's "Sonya Live" four months after the incident, holding forth on the supposed history of the rule and acclaiming the New Feminists for finally striking back: "I think we're undoing thousands and thousands of years of human history. You know the phrase 'rule of thumb' that everybody thinks is the standard measure of everything? It was a law in England that said you could beat your wife with a stick as long as it was no thicker ... than your thumb." 
Columnists and journalists writing about domestic violence were quick to pick up on the anecdote.
The colloquial phrase "rule of thumb" is supposedly derived from the ancient right of a husband to discipline his wife with a rod "no thicker than his thumb." (Time magazine, September 5, 1983)
A husband's right to beat his wife is included in Blackstone's 1768 codification of the common law. Husbands had the right to "physically chastise" an errant wife so long as the stick was no bigger than their thumb - the so-called "rule of thumb." (Washington Post, January 3, 1989)
Violence against women does not have to be the rule of thumb - an idiom from an old English law that said a man could beat his wife if the stick was no thicker than his thumb. (Atlanta Constitution, April 22, 1993)
The "rule of thumb," however, turns out to be an excellent example of what may be called a feminist fiction.  It is not to be found in William Blackstone's treatise on English common law. On the contrary, British law since the 1700s and our American laws predating the Revolution prohibit wife beating, though there have been periods and places in which the prohibition was only indifferently enforced.
That the phrase did not even originate in legal practice could have been ascertained by any fact-checker who took the trouble to look it up in the Oxford English Dictionary, which notes that the term has been used metaphorically for at least three hundred years to refer to any method of measurement or technique of estimation derived from experience rather than science.
Read all of Hoff Summers' article for a fascinating history of the origins of this myth and it's many political misuses.