“Rule of thumb” is an idiomatic expression for a guide that allows very quick calculations or gives short and simple rules for behavior. It is by no means a precise or hard-and-fast rule, but it does provide a general idea. For example, “as a rule of thumb, don’t wear black to a wedding.”
One wonders where the idiomatic expression originated. One very shocking theory was presented by Sharon Fenick, who said it came from a rule dating back to old England. Then, the English were “legally” allowed to beat their wives, children or servants—if the stick were thinner than one’s thumb. Historians have since disproven her arguments for that (and thankfully, the practice went into decline in the 1700s, as noted by William Blackstone’s “Commentaries on the Laws of England”).
“Rule of thumb” may have referred to the practice of using one’s hands or feet as “rough rulers.” (Hence the origins of the unit of measurement, “foot.”) In fact, Jonathan Swift speaks of a tailor’s rule of “twice around the thumb is once around the wrist” in his famous classic, “Gulliver’s Travels.” Could rule of thumb have been originally used to measure shirt cuffs?
Others believe it was the brewers who coined the phrase, after the habit of using one’s thumb to check the temperature of beer. Or the sailors, who used to lick the thumb and hold it up to check the direction of the wind.
None of these theories seem adequate, and it is most likely that the source of the idiomatic expression lie in none of them and has simply been lost to the ages.