What's Up Meaning

(rhetorical question, colloquial) A casual greeting with a meaning similar to how are you? or nice to meet you.

Example: 1884, Mark Twain, Huckleberry Finn:
  Hello, what's up? Don't cry bub. What's the trouble?
1912, W. Somerset Maugham, Lady Frederick: A Comedy in Three Acts, Act 1:
  Lady Frederick: Oh lord, I wish I were eighteen. [She sinks into a chair, and an expression of utter weariness comes over her face.]
  Gerald: I say, what's up?
  Lady Frederick: [Starting.] I thought you'd gone. Nothing.