1756, "special vocabulary of tramps or thieves," later "jargon of a particular profession" (1801), of uncertain origin, the usual guess being that it is from a Scandinavian source, and the dictionaries offer Norwegian slengnamn "nickname," slengja kjeften "to abuse with words," literally "to sling the jaw," related to Old Norse slyngva "to sling." But OED, while admitting "some approximation in sense," discounts this connection based on "date and early associations." Liberman also denies it, as well as any connection with French langue (or language or lingo). Rather, he derives it elaborately from an old slang word meaning "narrow piece of land," itself of obscure origin. Century Dictionary says "there is no evidence to establish a Gipsy origin." Sense of "very informal language characterized by vividness and novelty" first recorded 1818.
[S]lang is a conscious offence against some conventional standard of propriety. A mere vulgarism is not slang, except when it is purposely adopted, and acquires an artificial currency, among some class of persons to whom it is not native. The other distinctive feature of slang is that it is neither part of the ordinary language, nor an attempt to supply its deficiencies. The slang word is a deliberate substitute for a word of the vernacular, just as the characters of a cipher are substitutes for the letters of the alphabet, or as a nickname is a substitute for a personal name. [Henry Bradley, from "Slang," in "Encyclopedia Britannica," 11th ed.]
A word that ought to have survived is slangwhanger (1807, American English) "noisy or abusive talker or writer."