a slap on the wrist是指一种比较轻微的警告，只用到了“wrist手腕”，而没有用到hand。

【例句】
The fine was so low, it was little more than a slap on the wrist.
罚款很低，不过是微不足道的一点而已。

The attorney got her client off with a slap on the wrist.
这位律师使她的委托人仅受到轻罚就获释了。