“Slap on the wrist” is a mild punishment.
Example: The judge gave her a slap on the wrist by sentencing her to a few hours of community service.
Text in the clip from “Dexter: Do the Wrong Thing”:
– So what happened?
– Nothing. Guy gets a slap on the wrist. But, a couple of weeks later, he drops dead. Rat poisoning. Sound like a coincidence to you?
– Wow, I don’t know.
– Three makes a pattern. Maybe they all got in her way.