Let me put this right up front. The headline to this post is slightly misleading, but not dishonest. For the origin of the word slang indeed has been found, just not recently. It has been found for more than 100 years. The occasion for bringing this fact up now, and for the misleading headlinese, is that people still persist in presenting the origin as disputed. Anatoly Liberman, a highly respected etymologist who is in no way averse to hedging when warranted, tries to put the practice to rest in a recent Oxford University Press blog post titled "The Origin of the Word SLANG is Known!" That important exclamation point says not "exciting discovery!" but "Sheesh! Stop acting like it isn't known already!"
As Liberman explains, it goes back to an old use of slang for “a narrow piece of land running up between other and larger divisions of ground.” It's related to Scandinavian terms having to do with free movement and wandering, and the word's "route was from 'territory; turf' to 'those who advertise and sell their wares on such a territory,' to 'the patter used in advertising the wares,' and to 'vulgar language' (later to 'any colorful, informal way of expression')."
Liberman has made his life's work the thorough investigation of words marked "origin unknown" in etymological dictionaries. Though the explanation of slang laid out above appeared in his 2008 dictionary and is often credited to him, he gives credit to John Sampson, who set it forth in 1898. It was overlooked at the time, and Liberman laments,
"Few English words of disputable origin have been explained so convincingly, and it grieves me to see that some dictionaries still try to derive slang from Norwegian regional slengja 'fling, cast' or the phrase slengja kjeften 'make insulting allusions' (literarally 'sling the jaw'), or from the old past tense of sling (that is, from the same grade of ablaut as the past tense of sling), or from language with s– appended to it (even if the amazing similarity between slang and language helped slang stay in Standard English, for many people must have thought of some hybrid like s-language). All those hypotheses lack foundation. The origin of slang is known, and the discovery made long ago should not be mentioned politely or condescendingly among a few others that stimulated the research but now belong to the museum of etymology."