The phrase 'A Slap on the Wrist' means to get a light punishment (for doing something wrong).
Example of Use: "He should be in jail for what he did, but he got off with just a slap on the wrist."
If you’ve done wrong and are caught, you probably feel fortunate to get away with just a slap on the wrist. The origin of the idiom 'a slap on the wrist' is probably 18th century England, as it was in this place and time that the word 'slap' began to be used not just literally, but figuratively. As punishments were often quite severe in those days, a slap on the wrist would have been considered to be extremely minor.