Μάνγα is not a greek word, maybe you mean "μάγκα". It 's a way to call a friend. It 's like "dude" in english.

Μάγκας means a person who follows his own values and doesn't conform to preordered ways of life not obeying to leaders or to the common conceptions of humen.''Είσαι μάγκας' means you are a brave and authentic person who know who he is and that you are accepted from others because of this.Also,it's used as a common greeting when we meet a friend who is a man.

On the other hand "μάγκας" refers to a character , similar to rebetes - a social group in the Belle Epoque era... see: http://en.wikipedia.org/wiki/Mangas

Originally "μάγκα" is an Albanian word. It refers to a group of partisans during the Greek revolution of 1821 against the Ottoman Empire. It was the tiniest group of military organisation during these years. More information in Greek Greek Wikipedia: http://bit.ly/cUtWiI