Sharon Fenick first heard the figure of speech "rule of thumb" cited as a sexist pejorative during her freshman year at Harvard seven years ago.

The phrase was invoked in a lecture as an example of domestic abuse permitted by British common law. The rule of thumb, according to the professor, was a law that allowed a man to beat his wife so long as the rod used was no thicker than his thumb. But over the centuries, the term had evolved into vernacular for an "approximate measure."

"It sounded very believable to me," says the 24-year-old Fenick, now in her third year of law school at the University of Chicago. "I was having my first contact with feminist thought and [the explanation] was very impressive to me. It was one of those things I really remember that spread around. I can't remember when I found out it wasn't true."

Unlike Fenick, untold historians, feminists and legal experts are unaware that the folk etymology for "rule of thumb" is false. For them, the notion of a "rule of thumb" makes perfect sense, originating as it allegedly does from a legal system they see as misogynistic.

In January, wordsmith William Safire debunked "the rule of thumb's" false etymology in his New York Times Magazine column. The phrase had been called to his attention by the president of George Washington University, where a female student had denounced its use by an administrator remarking on budget problems in the student newspaper.

In gender- and women's-studies courses across the country, the phrase is still cited as an example of unconscious acceptance and tacit condoning of sexist policy. A computer search for the use of "rule of thumb" and "wife" in the same newspaper sentence reveals many letters to the editor in recent years from women irate about the casual appearance of the figure of speech in news articles. In a televised news analysis about domestic violence in 1994, even commentator Cokie Roberts noted the misconception.

The false etymology persists despite the Oxford English Dictionary definition: "A method or procedure derived entirely from practice or experience, without any basis in scientific knowledge; a roughly practical method." The OED dates the phrase's first reference to 1692.

In the Encyclopedia of Word and Phrase Origins, "rule of thumb" is additionally defined as a method by which brewers once tested the temperature of a batch of beer: They dipped a thumb in the brew.

During her first year of law school, Fenick, a wordsmith herself, was determined to unravel the history of the "rule of thumb." Did the phrase stem from a specific rule? Was there such a rule? Even if there wasn't a rule, did an infamous judge's ruling establish "thumbstick" guidelines for would-be wife beaters?

She discovered that while "rule of thumb" was not accepted law, there was evidence aplenty that the British legal system and the American legal system it inspired were unkind to women. "I found out that in the 1800s [wife beating] really was a debatable proposition," she says.

Wife beating is acknowledged in Blackstone's "Commentaries," and many court rulings sanctioned the practice. But whether the "rule of thumb" was accepted as law was a separate matter.

Fenick traced the earliest possible reference to the 17th century, when one Dr. Marmaduke Coghill, an Irish judge, held that a man who had beaten his wife "with such a switch as the one he held in his hand" was within his matrimonial privilege.

In the 18th century a judge named Francis Buller, dubbed "Judge Thumb" by the famous caricaturist James Gillray, was said to have allowed that a man could beat his wife, as long as the punitive stick was no thicker than his thumb. (A witty countess was said to have asked the judge to measure her husband's thumb exactly, so that she might know the precise extent of his privilege.)

Fenick also found three 19th-century cases in America that mention the "rule of thumb," including an 1868 ruling in North Carolina that "the defendant had a right to whip his wife with a switch no larger than his thumb."

Buller's "thumbstick" opinion and the three American rulings Fenick found were intriguing -- and damning -- but did not constitute definitive proof that the rule of thumb was derived from British common law.

As Fenick, encouraged by a law professor, considered publishing her findings, she found that Henry Ansgar Kelly, a University of California English professor, had beaten her to the punch. His "Rule of Thumb and the Folklaw of the Husband's Stick" appeared in the September 1994 Journal of Legal Education.

Kelly, much to Fenick's disappointment, had covered the same territory as she. (Although she proudly observes that his article overlooked the earliest reference to "rule of thumb" by Coghill.) Three and half years later, Safire would rely entirely on Kelly's article to make his case in his column.

Fenick's efforts were not in vain, however. In response to a query from a correspondent to the alt.folkore.urban newsgroup linked to the Urban Legends Web site, Fenick posted her article where it is now part of the site's permanent archives. Since its inception, the site has expanded its mission from probing the genesis and spread of urban legends to "confirming or disproving beliefs and facts of all kinds, including origin of vernacular."

"Rule of thumb" and other figures of speech can work much the same way that urban legends do: They may appear mysteriously, spread spontaneously and contain elements of humor or horror. And, like urban legends, a figure of speech may contain a grain of emotional, if not actual, truth.

Thus it was easy at first for Fenick and others to believe that the "rule of thumb" was founded in common law. Patricia A. Turner, a University of California at Davis folklorist, well understands how a falsehood can acquire the mantle of truth.

In "I Heard it Through the Grapevine: Rumor in African-American Culture," Turner examines the way allegations of forced birth control, corporate collusion with the Ku Klux Klan, drug distribution targeted at urban areas and other anti-black conspiracy theories circulating in the African-American community are based on racist realities and serve as a form of resistance against white oppression.

The same theory can be applied to the rule of thumb, Turner says. A text may be proved to be inaccurate or false, but "if it reflects some deeper truth in society, it doesn't go away." The term "rule of thumb" may "not have that specific etymological origin, but men have dominated women in workplaces and in homes and in virtually every setting. It speaks to a deeper truth."