From unknown (1756), from Scandinavian (?) 'sleng' ("a slinging; an invention of new words").
Very informal words and expressions that are more common in spoken language, especially used by a particular group of people.
Origin uncertain and much debated. The word appeared suddenly in the mid-18th century, initially meaning 'the special vocabulary of tramps or thieves.' One plausible theory derives it from a Scandinavian source: compare Norwegian 'sleng' (a slinging, an invention of new words), 'slengenamn' (a nickname), 'slengja kjeften' (to sling the jaw, to use abusive language). Another theory connects it to 'sling' in the sense of hurling words. The word itself is appropriately mysterious — slang tends to emerge from the margins, and its own origin is just as marginal and untraceable. Key roots: sleng (Norwegian (possible): "a slinging, an invention of new words").
Samuel Johnson refused to include slang words in his famous 1755 dictionary, calling them 'fugitive cant' — yet the word 'slang' itself appeared just one year later in 1756, as if in defiant response.