Can I really beat my wife with a stick no thicker then my thumb?
No, but the phrase does have an interesting history. There is a fairly widespread belief that the term "Rule of Thumb" originally applied to an old English common law that stated a man could beat his wife, provided that any rod used was no thicker than his thumb.
Rule of thumb, by today's standards is a rough guide for something, a simple method of approximation. The original phase is mostly credited to early woodworkers, as a measurement using ones thumb. The exact etymology has never be fully explained, but most people agree to the woodworkers origin.
English common law does say that it was legal for a man to “chastise his wife in moderation”, but there are no records of the “rule of thumb” law. This “law” has been passed down by word of mouth since the 17th century as a form of “folklore”.
But this folklore was so widely believed that it had made its way into courts!!!
There was a 1868 case, State v. Rhodes, where a husband was found innocent because, the judge said, "the defendant had a right to whip his wife with a switch no larger than his thumb,"
The 1874 case State v. Oliver (North Carolina Reports, Vol. 70, Sec. 60, p. 44) states: "We assume that the old doctrine that a husband had the right to whip his wife, provided that he used a switch no larger than his thumb, is not the law in North Carolina."
Both of these cases helped elevate the folklore surrounding the phrase...
And then the press does not help dispel the myth either.
The popular press writing about domestic violence have been quick to pick up on this phrase.
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)
So lets look at this old English common law that everyone is talking about. This usually refers to William Blackstone's(1723-1780) "Commentaries on the Laws of England". Blackstone took centuries of laws and customs and created a clearly organized tome (multi-volume books). This was the basis for much of the early U.S. Law. No where in the books does Blackstone describe the rule of thumb, but one interesting find is this;
"The husband ... by the old law, might give his wife moderate correction. For, as he is to answer for her misbehavior, the law thought it reasonable to intrust him with this power of restraining her, by domestic chastisement, in the same moderation that a man is allowed to correct his apprentices or children.... But this power of correction was confined within reasonable bounds and the husband was prohibited from using any violence to his wife.... But with us, in the politer reign of Charles the Second, this power of correction began to be doubted; and a wife may now have security of the peace against her husband.... Yet [among] the lower rank of people ... the courts of law will still permit a husband to restrain a wife of her liberty in case of any gross misbehavior." 
So no reference to Rule of Thumb in William Blackstone's treatise on English common law. It should be noted that, British law since the 1700s and American laws predating the Revolution have prohibit wife beating.
The phrase "rule of thumb" has nothing to do with beating your wife and is most likely etymology is a form of measurement.
Meaning: A basic rule that is usually but not always correct.
Example: As a rule of thumb, plant tomato seeds three inches deep.
1. Sir William Blackstone, Commentaries on the Laws of England (New York: W. E. Dean, 1836), vol. 1, p. 36.