What does "slang" mean?

Additional senses

1. 2.Any long, narrow piece of land; a promontory. [Local, Eng.] Holland.
2. 3.A fetter worn on the leg by a convict. [Eng.]
3. 4.Low, vulgar, unauthorized language; a popular but unauthorized word, phrase, or mode of expression; also, the jargon of some particular calling or class in society; low popular cant; as, the slang of the theater, of college, of sailors, etc.
4. 5.To address with slang or ribaldry; to insult with vulgar language. [Colloq.] Every gentleman abused by a cabman or slanged by a bargee was bound there and then to take off his coat and challenge him to fisticuffs. London Spectator.