Cambridge Dictionary对“rule of thumb”的定义为“a method of judging a situation or condition that is not exact but is based on experience”，就是一种凭借经验对情况作出判断的方法，可以译为“大拇指规则”或“经验规则”。这个短语可以追溯至十七世纪，常用于数量计量，因为人们通常是通过对比大拇指的宽度或长度来进行度量。有一种说法是，“rule of thumb”与英国的一项普通法规定有关，下面看看Wikipedia对此项说法的解释：

“A modern folk etymology holds that the phrase is derived from the maximum width of a stick allowed for wife-beating under English common law, but no such law ever existed. This belief may have originated in a rumored statement by 18th-century judge Sir Francis Buller that a man may beat his wife with a stick no wider than his thumb. The rumor produced numerous jokes and satirical cartoons at Buller's expense, but there is no record that he made such a statement.

English jurist Sir William Blackstone wrote in his Commentaries on the Laws of England of an ‘old law’ that once allowed ‘moderate’ beatings by husbands, but he did not mention thumbs or any specific implements. Wife-beating has been officially outlawed for centuries in England (and the rest of the United Kingdom) and the United States, but continued in practice; several 19th-century American court rulings referred to an ‘ancient doctrine’ that the judges believed had allowed husbands to physically punish their wives using implements no thicker than their thumbs.”

从上面的描述可以看出，“rule of thumb”这一短语其实与家庭暴力毫无关系，但是，仍有人在著作中将二者联系起来，如下所示：

“The phrase rule of thumb first became associated with domestic abuse in the 1970s, after which the spurious legal definition was cited as factual in a number of law journals, and the United States Commission on Civil Rights published a report on domestic abuse titled "Under the Rule of Thumb" in 1982. Some efforts were made to discourage the phrase, which was seen as taboo owing to this false origin. During the 1990s, several authors correctly identified the spurious etymology; however, the connection to domestic violence was cited in some legal sources even into the early 2000s.”

例句：

原文：

As a rule of thumb, the ice on the lake should be at least two inches thick to support one person.

译文：

根据经验，湖上的冰至少达到两英尺厚才能承受一个人的重量。

来源：

Cambridge Dictionary；

Wikipedia