There are all manner of false word origins that get tossed around the web, but one of the more common ones I see is that “slang” is short for “shortened language,” and while it’s believable, there’s no historical record to indicate that this was ever true.
(This supposed origin also doesn’t entirely make sense because slang terms aren’t necessarily shorter than the terms they replace. Example: “too big for [one’s] britches” replaces “arrogant” or “cocky.” On the contrary, the factors required for colloquialisms to qualify as slang are that they are informal, and they are specific to a particular social group or culture.)
But the word’s true origin (or at least the most likely one) is even more interesting, in my opinion:
Popularized in English during the mid-18th century, “slang” referred specifically to the lexicon of tramps and thieves. It is most likely Nordic in origin, derived from the old phrase slengja kjeften, which literally meant “to sling the jaw,” but which carried the implication “to abuse with words.”
Slang’s current meaning—”informal but vivid colloquial speech used as a deliberate substitute for other terms or concepts in the same vernacular”—became common in the early 19th century, growing in popularity around the same time as the word “slangwhanger,” an American English term meaning “one who uses abusive slang” or “a ranting partisan.” Tragically, “slangwhanger” is obsolete in our current lexicon.