What's up with "What's up?"

Today in passing, I asked Matt "What's up?"
"I don't know," he replied with a straight face.

This started a conversation about the origions of the phrase "What's up" and it's meaning. I didn't find anyone claming to know exactly where the phrase started, but many attribute the popularity to Bugs Bunny. Here are some fun facts for you.

'Eh, What's up Doc?' joins 'That's All Folks!' as the best-known lines from Tex Avery's Looney Tunes cartoon series. It was delivered by Bugs Bunny, while nonchalantly chewing on a carrot, in most of the cartoons in which the character appeared, beginning with A Wild Hare, 1940. This was the first Bugs Bunny cartoon, although Bugs wasn't named until the second cartoon - Elmer's Pet Rabbit ('Happy Rabbit', a prototype Bugs Bunny with a somewhat different personality had appeared earlier).
Avery explained how the line became established in the numerous cartoon confrontations between Bugs Bunny and the hapless hunter Elmer Fudd:
"We decided he [Bugs] was going to be a smart-aleck rabbit, but casual about it. That opening line of 'Eh, what's up, Doc?' floored them. They expected the rabbit to scream, or anything but make a casual remark. For here's a guy pointing a gun in his face! It got such a laugh that we said, 'Boy, we'll do that every chance we get.'"
Chuck Jones went on to explain that the demeanor of Bugs when delivering the line was adapted from Clark Gable's performance in It Happened One Night. In that film, Gable's character leans against a fence eating carrots and gives instructions with his mouth full to Claudette Colbert's character. The scene was well-known to audiences at the time who would have been well aware that Bugs was spoofing Clark Gable.
The line has outlived Bugs Bunny and is now commonly used worldwide as a jokey alternative to the straightforward query 'what's up?', i.e. 'what's going on?'.
http://www.phrases.org.uk/meanings/406400.html