Beauty of Slangs

While watching the Mavs-Kings game, I saw a fan holding up a sign that said, "Nick Van 'Da Man' Excel." I thought about that time when Dad was mystified to see a memo from his boss, which said, "You are the man!" I tried to convey the actual meaning to him for about 2 minutes and then just gave up. I could only tell him his boss thinks he's doing a great job.