slap on the wrist

(noun) a very light punishment

Example Sentences:

A: Did you get in trouble for starting that fire?
B: Nope! I got a slap on the wrist!

The bank robber escaped with aslap on the wrist when the judge sentenced him to 10 days in jail.

From the Blogs:

Tobogganing On Duty? Slap On The Wrist

Media:

Similar idioms:

1. scot-free
2. make light of
3. fight fire with fire
4. give someone the green light
5. get along like a house on fire