A severe reprimand.
A dressing down is a strong verbal attack on a persons actions, explaining how they messed up, and making it quite clear that they are in the doghouse.
On the other hand, dressing down can also mean dressing in a more sloppy or relaxed manner than is usual, either for that person or for the situation the person is in. To wear clothes that will give people the impression that you are slovenly and fashion impaired. | 
