What does "dress down" mean?

Scold, reprimand, as in The sergeant will dress down the entire unit. In the 15th century the verb dress alone was used in the sense of “punish,” down being added several centuries later. It also gave rise to the noun dressing down for punishment with blows or words. For example, The teacher gave the girls a severe dressing down.

Wear informal clothes, as in It's best to dress down for a party like a barbecue. [Mid-1900s] For the antonym, see dress up.