means Slang is the informal, playful, ever-shifting vocabulary of a particular group or moment, often invented to mark who's in the know and who isn't.

from The word surfaces in 18th-century English meaning the special jargon of thieves and other disreputable types, but its own roots are genuinely murky — etymologists shrug. A favorite guess links it to Scandinavian words like Norwegian dialectal 'slengja' (to sling, to fling), as if slang were language flung sideways out of the mouth; but this is unproven, and the honest answer is that the origin of 'slang' is, fittingly, slang's own secret.