Life is Silly

Some etymologies are part of urban legends, many of which allege a scandalous origin for a common and innocent word. One common example has to do with the phrase rule of thumb, meaning a rough measurement; the width of adult male thumb is roughly one inch. An urban legend has it that the phrase refers to an old English law under which a man could legally beat his wife with a stick no thicker than his thumb (though no such law ever existed). [1] Interestingly, the phrase "rule of thumb" is known in Finland (which is a Metric country) as "nyrkkisääntö" (rule of fist); the width of adult male fist is roughly ten centimeters). Certain feminists have interpreted this to mean that a man had been legally allowed to beat his wife with his fists but not with a weapon.
-http://en.wikipedia.org/wiki/False_etymology