There’s a very common usage in fashion/beauty, “the latest thing in fashion,” which also refers to a “phenomenon” or “trend.” The use goes back at least to 1862(!!), where I found it in this article from Peterson’s, regarding “the latest thing in opera cloaks.”
There’s a possible usage in a medical journal that goes back a little further, from the London Medical Gazette in 1842: “The late Doctor Warren told me that the use of opium was the latest thing he learned in practice.” But I think the usage is ambiguous–the “latest thing” in this quote seems literal: the most recent process that Warren learned. Still, taking in the broader reading from the article itself, which is discussing the increasing use of certain procedures, it could also mean “phenomenon/trend.”
So… let’s say mid-1800s (in English) or possibly earlier?
Using the fashion connection, it also feels quite possible that it’s a phrase that might be found in French that became translated into a more bland “thing” in English.
Well, we could go a couple centuries earlier, (and for the Dope, I’m a bit surprised that nobody has mentioned it yet):
There once was a fairly well-known guy who said: “the play’s the thing”.
(Hamlet,Act 2, Scene 2)
But that’s not what the OP is asking.
The real answer is the 1990’s, as explained in Andy L/s link above.
Before then, the word thing was used in many contexts, as other posts have mentioned.But not meaning “a new and previously unknown social phenomenon”.
But the most important part of the OP’s question isn’t the word "thing’–it’s the word “a”.
The expression, using both words, became common in American speech during the 1990’s.
He said “the play’s the thing wherein I’ll catch the conscience of the king”. That’s quite a different usage - not “thing” as generally prevelant fad, trend, phenomenon, etc., but “thing” as in particular object that I will utilise for a stated purpose.