Slang is vocabulary that is used between people who belong to the same social group and who know each other well. Slang is very informal language. It can offend people if it is used about other people or outside a group of people who know each other well. We usually use slang in speaking rather than writing. Slang normally refers to particular words and meanings but can include longer expressions and idioms.
Slang changes quickly, and slang words and expressions can disappear from the language. For these reasons, it is generally best for learners of English to avoid using slang.
Some current examples:
He’s a geek. (someone who is felt to be strange because they spend all their time studying)
[teenager speaking about some elderly people in a park]
Look at those oldfogeys on the bench!
It’s all gone pear-shaped. (wrong, not as we expected)