Wiktionary
vb. (context gay slang English) To subtly insult someone.
Wikipedia
The expressions "throw shade, "throwing shade", or simply "shade", are slang terms used as insults. Miriam-Webster.com defines "shade" as "subtle, sneering expression of contempt for or disgust with someone—sometimes verbal, and sometimes not" . Oxford Dictionaries defines "throw shade" as a phrase used to "publicly criticize or express contempt for someone".
Usage examples of "throw shade".
By now the young men of the house had brought food, drinks, and pitchers of water up onto the roof, and they were also busy arranging parasols to throw shade on a nest of chairs and small tables.
Even the channel of the path was enough to throw shade, and the very stones of the road cast tapering dashes of darkness westward, as long as Jael’.