[idiom] a slap on the wrist

slap _
noun >a quick hit with the flat part of the hand or other flat object  :
verb  >to hot someone with the flat part of the hand or other flat object :

• She slapped his face. 
• His friends slapped him on the back when he said he was getting married.
• 'Congratulations!' he said, slapping me on the back. 

wrist [rist] _ noun 

>the narrow part of the arm above the hand that can bend to move hand in different directions:

• He developed strong wrists playing baseball.

a slap on the wrist

meaning : If someone gives you a slap on the wrist, the give you a mild punishment for making a mistake or doing something wrong. 

•  The tribunal didn't think it was a serious offence, so they just gave him a slap on the wrist. All he got was a warning.
• Some of out players got a slap on the wrist for getting to the training session late. The coach warned them not to be late again and then made them do twenty push-ups.