We have all heard the phrase "rule of thumb" but actually, does that make any sense? And whose kingdom is the thumb ruling anyway?
Well first off, in this instance, the use of the word "rule" refers to measurements, not laws, much like the word "ruler". The phrase can be summed up as "a general principle or rule based on experience or practice, as opposed to a scientific calculation." Personally, I like the word guesstimate, so I'm just going to chuck that in there too.
It is quite commonly believed (though I'm sure not by you, dear reader) that the phrase rule of thumb dates back to an English law that allowed husbands to beat there wives as long as the stick wasn't wider than their thumb. While we won't debate the merits of such a law on these pages, this is certaintly not the origin of the phrase. English law did allow for "moderate correction" and domestic violence was probably quite common but they aparently didn't go around measuring sticks with thumbs beforehand.
Instead, the origins of the phrase appear to be more mundane. First recorded in 1692, and already in common usage well before then, the phrase probably originates from some form of measurement, such as when planting seeds or woodworking. This distance is roughly one inch and in many European languages the word for inch is same as the word for a thumb.