Origins
The word 'slang' is one of the great etymological mysteries of the English language — a word whose origin is as elusive and disreputable as the vocabulary it describes. It appeared suddenly in the mid-18th century, initially meaning the special cant of thieves and vagabonds, and within a century had expanded to cover any informal vocabulary that thumbs its nose at standard usage. Despite centuries of scholarly investigation, no one can say with certainty where it came from.
The most plausible theory connects 'slang' to Scandinavian sources. Norwegian has 'sleng' (a slinging, a throwing), 'slengenamn' (a nickname, literally a 'flung name'), and the phrase 'slengja kjeften' (to sling the jaw — to use abusive or coarse language). Swedish dialect has 'slangord' (a slang word). The idea would be that slang is language that is 'slung' — thrown out carelessly, tossed off without the deliberation that formal speech requires. This etymology has the virtue of matching slang's character: slang is linguistically flung, hurled, slung rather than carefully placed.
Other theories have been proposed. One suggests a connection to 'sling' through the idea of hurling words. Another, now largely discredited, derived it from 'thieves' language' or 's language' (the language of the 's,' meaning the streets). A particularly creative suggestion links it to 'slang' as a variant of 'sling' in the sense of a travelling salesman's route — 'slang' being the patter used by hawkers and peddlers. None of these can be confirmed. The word keeps its secret, which feels entirely appropriate.
Latin Roots
What is certain is the social history. When 'slang' first appeared in print around 1756, it described the cryptolect of the criminal underworld — the specialized vocabulary that thieves, beggars, and prisoners used to communicate without being understood by outsiders. This was also called 'cant,' 'argot,' or 'flash language.' Early slang dictionaries, such as Francis Grose's Classical Dictionary of the Vulgar Tongue (1785), treated their subject matter with a mixture of fascination and moral disapproval, presenting the vocabulary of the streets as a kind of anthropological curiosity.
By the 19th century, 'slang' had expanded beyond criminal vocabulary to encompass any markedly informal language. Schoolboy slang, military slang, sporting slang, theatrical slang — every social group was recognized as generating its own informal vocabulary. The word lost its exclusive association with criminality and gained a broader, though still faintly disreputable, meaning. To use slang was to be deliberately casual, to signal membership in a group, to reject the formality of standard English in favor of something more vivid, more immediate, more alive.
Linguistically, slang performs several crucial functions. It creates in-group solidarity — if you know the slang, you belong. It provides expressive intensity — slang terms for common concepts are often more vivid and emotionally charged than their standard equivalents. It enables linguistic play — the creation and adoption of slang is one of the most creative activities in any language, involving metaphor, metonymy, clipping, blending, and semantic shift. And it drives language change — many words that are now perfectly standard English began as slang. 'Bus' was slang (clipped from 'omnibus'), 'mob' was slang (clipped from 'mobile vulgus,' the fickle crowd), and 'banter,' 'fun,' and 'stingy' were all condemned as vulgar slang when they first appeared.
Legacy
The word 'slang' thus names one of language's most vital and least controllable forces — the constant bubbling up of new informal vocabulary from below, driven by creativity, group identity, and the universal human desire to say familiar things in fresh ways. That such a powerful force should be named by a word of uncertain origin, arriving suddenly from the margins, is perhaps the most fitting etymology of all.