Throw shade is a popular phrase that means to talk trash or show disrespect. Per Ru Paul, the act requires creativity.
Generally speaking, one throws shade publicly, not privately. Whether or not this is cruel and totally not cool is a divided opinion of the Internet, but now news headlines are easier to understand.
Throw Shade, Urban Dictionary
RuPaul Explains the Difference Between Throwing Shade and Being a Straight Up Bitch, Jezebel