The idiom rule of thumb, meaning a principle that’s widely useful but not strictly accurate in all circumstances, has origins in the practice of making measurements with one’s thumb.1 In this idiom, rule originally carried one of its now little-used definitions—a straight-edged device used for measuring or drawing lines—but it’s now taken on the more common definition. Now, when you hear the phrase rule of thumb, it’s more or less synonymous with general rule.
Contrary to the old myth now widely repeated on the web, rule of thumb‘s origins have nothing to do with wife-beating, so the idiom is not inherently offensive (though the fact that some people think it is offensive might be cause to use it with caution).2
A good rule of thumb is to use less than 10% of the credit that’s available to you. [The Takeaway]
A key rule of thumb to follow when the snowstorm of the century bears down upon you is to not drink too much red wine the first night of the blizzard. [Market Watch]
Good rule of thumb: If they offer you a role that requires messed-up teeth, the role’s probably not that good, anyway. [San Francisco Chronicle]