HENRY VIII DID NOT KNIGHT A LUMP of steak to create “Sir Loin”; Thomas Crapper (1836-1910), despite his worthy work popularising the flush lavatory, was born several hundred years after the word “crap” was coined; “kangaroo” is not the Aboriginal for “I don't understand what you are saying”, supposedly the response when Captain Cook asked an Australian native to name the curious leaping creature he first saw in Queensland in 1770. (Gangurru means “grey kangaroo” in the Guugu Yimithirr tongue.) And Welsh rarebit derives from the word rabbit, even though it contains no rabbit, and doesn't come from Wales.

All of which is rather disappointing, since the false but widely-accepted story of a word's origin is often so much more colourful and interesting than the truth. Sometimes false etymologies are deliberately forged to serve a political or social purpose, but mostly they seem to emerge spontaneously, providing a psychologically or emotionally satisfying explanation for the birth of a word, irrespective of reality.

As a rule of thumb, the more elaborate and widely accepted the explanation, the more likely it is to be wrong.

Take “rule of thumb”. For decades, it was widely believed to refer to the practice of wife-beating. One standard women's studies textbook in the US states unequivocally “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... [it] essentially allowed a man to beat his wife without interference from the courts”.

A pretty nasty little phrase, then, to be deployed with extreme caution and sensitivity. Except that “rule of thumb” has no such origin: it has never appeared in any statute book, and does not originate in legalised domestic violence. The first joint of the thumb is approximately an inch in length, so the rule of thumb allowed joiners to make rough measurements (the use of “hand” to measure the height of a horse is similar). It may also derive from the beer trade, as brewers traditionally tested the temperature of a batch of beer by sticking their thumb in.

Though no fault of its own, this innocent phrase became a semiotic battleground, with feminists using a false etymology as a stick to beat male chauvinists, and chauvinists debunking the word's false origins as a way of getting back at feminists.

The Nazis claimed, falsely, that the word “Slav” was related to “slave”, providing a built-in etymological excuse for oppressing and enslaving Slavic peoples. Enemies of the Dominican order (founded by Saint Dominic in the 13th century) falsely claimed that their name derived from domini canes, “god's dogs”. Even melted cheese has been deployed for oneupmanship: “Welsh rabbit” (which evolved, for no clear reason, into “rarebit”) was a cheap and meatless meal, dating back to when Welshness was a byword for extreme poverty. Cheese on toast became known by an ironic term, as a dig at the Welsh.

Most false or folk etymologies, are not so overtly political, seeming to evolve from pure pleasure in a good story and an agreeable explanation. The myth of sirloin, for example, may date back to a pun made by the poet John Taylor in 1630, when he referred to a character who would “presently enter combate with a worthy knight called Sir Loyne of Beefe”. The joke was so good that it has run and run, and the act of knighting a lump of meat has been attributed, with authority but entirely without basis, to at least three kings: Henry VIII, James I and Charles II. The truth is less tasty: the word derives from the French sur, “on top” or “above”, to describe the top cut of the loin.

“Marmalade” was apocryphally used by Mary Queen of Scots to treat headaches. Whenever she tucked into a jammy confection of sliced citrus fruit, her French-speaking servants would suppos-

edly observe: “Marie est malade” (Mary is ill), corrupted into “marmalade”. A great story, but sadly only that: marmelo is Portuguese for quince, the original fruit used to make the stuff.

The human tendency to create and believe false origins for words is as old as language itself, a way to demystify and control the word, to stamp it with additional, local meaning. “OK”, for example, can be held to be German (from Oberst Kommandant, meaning “Colonel in command”) or Finnish (from oikea, meaning “correct”) or Scots (from “och aye”), or West African (from o ke, in Mandingo, meaning “certainly”.) OK was probably coined in Boston as shorthand for “orl korrect”, but the word is now so universal that any and all may claim the word by dressing it up with a relevant backstory.

In some ways, false etymologies — etymytholgies, as they are sometime known — are like family anecdotes. They may not literally be true; indeed, they may not be true at all; yet they contain a deeper kind of truth. The true origins of words is the archaeology of human culture; what we choose to believe about the birth of words can be just as revealing.

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• The 100 Best Books of the Decade
• The Times picks the 50 best paperbacks of 2009
• The conversation: James Ellroy