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 stillpoppin' out all over the place!
The act of forgetting that it is dress down day and arriving in your regularwork wear.
Joey turned up to work in his normal work clothes whilst all his colleagues had remembered to come in casual clothing, Poor Joey has some kind of Dress Down Syndrome.
A person you hate intensely not because they deserve such hatred but because they serve as a focus for your general hatred. A healthy way to dispense with excess animosity.
Jake's not really that big an asshole. He just serves as my hate sink.