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里面已经写得很清楚，表示一个粗略的原则，毕竟打老婆的时候不会仔细丈量自己用的棍子的粗细到底有没有超过自己的拇指。
在书中它是这么用的：
The rule of thumb is that...
我想应该翻译成：大体的原则是...