Throwing shade means expressing criticism or contempt indirectly, often through an apparently casual remark, a backhanded compliment, or a look.[1]
Ballroom usage
Dorian Corey explains shade and reading in Paris Is Burning, a documentary about New York’s ballroom scene. A read can make the insult plain. Shade can leave the listener to put it together.
Example
“You’re brave for wearing that to the wedding.” The sentence is a compliment right up until the word “that.”