The renowned scholarly institution UrbanDictionary defines the term as follows:
throw shade: to talk trash about a friend or aquaintance, to publicly denounce or disrespect. When throwing shade it's immediately obvious to on-lookers that the thrower, and not the throwee, is the bitcy [sic], uncool one
And also:
throwing shade: Throwing Shade, is to throw "attitude."
And finally:
shade: acting in a casual or disrespectful manner towards someone/dissing a friend
However, in researching more of where the term came from (apparently the black and Latino gay subculture in the 1980s), I came across this post that suggests that the usage by people outside this subculture is all wrong:
Corey explains other voguing terminology, such as “reading” and “throwing shade.” To read is to insult imaginatively – in opposition to the blunt gay-bashing taunts of the straight world. Reading is gay-to-gay sparring. Thus, when two black queens call each other “black queen,” says Corey, “that’s not a read, that’s just a fact.”
Throwing shade is reading at a refined level; it’s the curve to the pitch. If someone says they won’t call you ugly because you already know, well, you just got thrown a shade. When enmity reaches fever point and pride is involved, it’s time for voguing. This is direct competition, when contenders take their fight to the ball floor: the equivalent of jousting, dueling or stepping outside the bar.
You’ll notice that throwing shade is defined in opposition to the directness and cruelty of gay-bashing. It’s more artfully executed, more dependent on constructing a veiled (or not-so-veiled) insult rather than relying on obvious crudities and innuendo. Throwing shade requires wielding your words like a rapier rather than a cudgel.
In other words, that it's not merely trash-talking or throwing attitude, but something far more specific. Unfortunately, they didn't really provide examples of the term "throwing shade" being used in a manner that was deemed correct, or provide a succinctly stated alternative definition.
With this in mind, what is the actual correct usage of the term "throwing shade"?
Best Answer
Like other words and phrases, slang doesn't operate under copyright protections that require future users to use the words or words in precisely the manner that its originators did. If such rules did apply, a judge might very well rule that the 1980s meaning of "throw shade" and "thrown a shade" that the OP cites violated the commonly understood sense of "throw a shade" from the early 1900s. From Francis March, A Thesaurus Dictionary of the English Language (1902):
That being said, a Google Books search finds several discussions of "throw shade" from the mid-1990s that refer to what the OP identifies as the original slang sense of the term. Connie Eberle, Slang and Sociability: In-group Language among College Students (1996) includes this brief comment on "throw shade":
Eberle seems a bit quick to conflate gay slang and African American slang into one "distinctive vocabulary," and her implicit notion that college students as a class have little overlap with either homosexuals or African Americans is rather astonishing. Nevertheless, her understanding of the source and sense of "throw shade" in college slang during the 1990s may well be accurate.
Two other Google Books matches focus on the gay African American subculture from which "throw shade" in its modern sense seems to have arisen. Both articles are well worth reading at fuller length if you're interested in the subject. From Tricia Rose, "An Interview with Willi Ninja," in Microphone Fiends: Youth Music an Youth Culture (1994):
And from E. Patrick Johnson, "SNAP! Culture: a Different Way of Reading" in Text and Performance Quarterly 15(2) (1995), reprinted in Performance: Media and Technology (2003):
As an example of how uncontrollable slang is, however, we have this discussion in Eve Oishi, "Reading Realness: Paris Is Burning, Wildness, and Queer and Transgender Documentary Practice", in A Companion to Contemporary Documentary Film (2015), most of which consists of a description of children's slang that was in use by 1993:
Evidently, by 1993, children in schoolyards somewhere in the United States had already substantially altered the notion of shading to mean "verbal sparring between equals," not "silent disrespecting of a disliked person," as in the previous two examples. In fact, it isn't altogether clear that the schoolyard sense of the two terms wasn't the original sense of those terms. But that's the crucial feature of slang: It emerges by word of mouth—without a dictionary definition to tie it down, so it's hardly surprising that different understandings of a particular term may exist in different places at the same time. And over time, of course, the picture can get even blurrier.