Meaning
A means of estimation made according to a rough and ready practical rule, not based on science or exact measurement.
Origin
This
has been said to derive from the belief that English law allowed a man
to beat his wife with a stick so long as it is was no thicker than his
thumb.
Meaning里面已經寫得很清楚，表示一個粗略的原則，畢竟打老婆的時候不會仔細丈量自己用的棍子的粗細到底有沒有超過自己的拇指。
在書中它是這么用的：
The rule of thumb is that...
我想應該翻譯成：大體的原則是...