It's a German word mostly used in or around Hamburg which has absolutely nothing to do with the word nigger!
It's said that it was used by dockers in Hamburg for the first time and was later adopted by the Hamburger Hip-Hop Wave in the early 90s!
It has the same meaning as the English "mate", "dude" or "bro" depending on the context!
"Digga, hast du ein Problem?!" - "You got a problem, dude?"