Pronounced with an extra long 'nnnnn' to prevent unnecessary confusion, this term is used to refer to a friend or acquiantance who knows about drugs. Any type of drug. Why the gov'ment is corrupt and should legalise them. Which you should stick in your eyes. Which you should stick in your rectum. And so on.
Person A: Dude, smoking weed is boring, can we stick it in our eyeballs?
Person B: Don't know man, you better ask Joe, he's the Mann.
Person A: The man?
Person B: No, the MANN.
Person A: Joe, can we stick weed in our eyes?
Joe: By all means, but it won't get you high. Not a bit. Fancy a joint?
Person A: Thanks Joe, you're the Mann.
Joe: I know, man.