A rough estimate ie, a way of assessing, measuring or determining based on past experience, rather than precise scientific calculation.
The phrase itself has been in circulation since the 1600s. The earliest known use of it in print appears in a sermon given by the English puritan James Durham and printed in Heaven Upon Earth, 1685:
"many profest Christians are like to foolish builders, who build by guess, and by rule of thumb, (as we use to speak) and not by Square and Rule."
It is said to have originated in England, where a man could beat his wife with a stick as long as it is was no thicker than his thumb.
In 1782, Sir Francis Buller - a reputedly arrogant Judge, notorious for his unreasonably harsh punishments - is supposed to have ruled this practice legal.
He soon became known as 'Judge Thumb' after being portrayed satirically in a cartoon showing a man beating a fleeing woman, while Buller - carrying two bundles of sticks - looks on.
The caption read "thumbsticks - for family correction: warranted lawful!".
The phrase itself has been in circulation since the 1600s. The earliest known use of it in print appears in a sermon given by the English puritan James Durham and printed in Heaven Upon Earth, 1685:
"many profest Christians are like to foolish builders, who build by guess, and by rule of thumb, (as we use to speak) and not by Square and Rule."