It appears in Mark Twain's 1885 novel, "The Adventures of Huckleberry Finn." It's set around the 1850s, and Mark Twain makes a point saying he's rendered all the dialog accurately, so we can assume it was used in the 1850s. 'He stirred up in a kind of a startlish way; but when he see it was only me he took a good gap and stretch, and then he says: "Hello, what's up? Don't cry, bub. What's the trouble?"'

"What's up" is using as a conversation start between two person and its informal. I'm not sure about the origin but as you can understand "up" is likely refer to most important news or top headline and because of the whole slang is informal, it refer to your personal news like your mood or whatever.