a slap on the wrist不是在手腕上打一下，引申意思是“象征性的惩罚、轻微的惩罚”。

【例句】
The fine they gave her is just more or less a slap on the wrist.
他们对她开出的罚金不过是一个轻微的警告而已。

I hoped the judge would send this guy to jail. But all he gave him was a slap on the wrist.
我希望法官会把这个人送进监狱。但他只是象征性地对他进行了惩罚。