According to the Cambridge Dictionary, slang is "very informal language that is spoken rather than written, and used especially by particular groups of people."

Slang is vocabulary that is used between people of a social group, this group can be anything from a group of friends to the army or even the internet. It is extremely informal, and can be offensive to people when spoken outside of the group, or in formal situations. Slang is usually used in speaking, and barely in writing. It can refer to words, expressions and idioms. 

Slang has its own dictionary, even though it is a bit dated: A Dictionary of Slang and Unconventional English, which is slightly outdated, being from the 1930's, or the modern version: Urban Dictionary, where you can find the most interesting new words to use. Whenever I go there, there are a bunch of new words for me to explore. Slang has gotten even bigger since the rise of the internet, with online forums and boards coining new words every single day. 

For example, the word of the day for April the Third, Pretendonitis, "A made up injury to get out of doing something". 

And that is just what slang is. Language that is used in the moment, some of it outdated and forgotten within a year, and some remembered decennials later.

But how did slang start existing, and where does the word come from?