1. to dress less formally than normal at work or a gathering
2. to reprimand
How much are you going to dress down at the party?
In addition to, and to clarify, the other definition:
A person (often female) is dressed down when she is wearing clothes that do not accentuate her voluptuous parts (if any).
Despite her attempts to dress down by wearing long pants and a coat, that shit was still poppin' out all over the place!