Meaning "…an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination."
"The exact origin of the phrase is uncertain.The earliest known citation comes from J. Durham’s Heaven upon Earth, 1685, ii. 217: "Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb."
Sir William Hope wrote in his The Compleat Fencing Master, 1692: "What he doth, he doth by rule of Thumb, and not by Art." James Kelly's The Complete Collection of Scottish Proverbs, 1721, pg 257, includes: "No Rule so good as Rule of Thumb, if it hit."
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