This one ladies, feminists, and decent human beings – you are not going to like. This idiom is commonly thought to be derived from a law that limited the maximum thickness of a stick with which it was permissible for a man to beat his wife under English Common law.
Common law permitted a man to give his wife ‘moderate correction’ and ׳Rule of thumb׳ was apparently an unwritten law.
Bonus fact: Idioms and expressions grow in a cultural context and environmental background and sometimes continue to exist even after that background disappears. A contemporary example of this is the phrase ‘the penny has dropped’, which originated from a time when public telephone activation was done by a token, a period that ended in the late 20th century.
Some expression consists of a reference to a particular country or city, which is attributed to a particular attribute. Examples:
In English, we say: ‘It’s all Greek to me’, a phrase that says to mean no sense at all. A similar phrase appears in different languages, but in each country the ‘misunderstood’ language that is used as a metaphor is different. In French, for example, is C’est de l’hébreu, which means ‘this is Hebrew’.
The same goes for food. For example, Turkish coffee is called in Greece ‘Greek coffee, And what is known in most countries as American bagel referred to by Americans as Jewish bagel.