Real Dictionary

dress down
phrasal verb: If you dress down, you wear clothes that are less smart than usual.

dress down in American English
to reprimand; scold
to scold severely; reprimand

dress down in British English
to reprimand severely or scold (a person)

dress-down in American English
of or relating to a policy allowing casual attire in an activity, job, etc. that ordinarily requires more formal dress