The expressions "throw shade, "throwing shade", or simply "shade", are slang terms used as insults. Merriam-Webster defines "shade" as "subtle, sneering expression of contempt for or disgust with someone—sometimes verbal, and sometimes not" . OxfordDictionaries.com defines "throw shade" as a phrase used to "publicly criticize or express contempt for someone".
History
The first known use of "shade" is in the documentary film Paris Is Burning (1990), which is about the mid-1980s drag scene in Manhattan.
The expression was popularized by the American reality television series RuPaul's Drag Race. In 2015, Anna Holmes of The New York Times Magazine wrote:
Shade can take many forms — a hard, deep look that could be either aggressive or searching, a compliment that could be interpreted as the opposite of one. E. Patrick Johnson, who teaches performance studies and African-American studies at Northwestern University, and who has written about the tradition of insults in the gay and black communities, explains: "If someone walks into a room with a hideous dress, but you don’t want to say it's hideous, you might say, 'Oooh … look at you!’'" At its most refined, shade should have an element of plausible deniability, so that the shade-thrower can pretend that he or she didn't actually mean to behave with incivility, making it all the more delicious.