“Lunfardo” is a vocabulary that was born in Buenos Aires at the turn of twentieth century. It was a slang made up by criminals. They used it to talk without being understood by the police. Many lunfardo words come from different languages (such as italian, french or greek) because, at that moment, Buenos Aires received a huge inmigration from all around the world.
Tango, just like lunfardo, was born among the lower social classes. That´s why its lyrics are full of lunfardo slang. Later on, when tango became more and more popular, these words became part of every porteño´s vocabulary, even today.
Here there´s a list of the most frequently used lunfardo words nowadays. You might have heard them more than once.