"What's up doc?" - Is Bugs Bunny impersonating someone?

The (co-)Creator of Bugs Bunny, Tex Avery, once said:

We decided he was going to be a smart-aleck rabbit, but casual about it, and his opening line in the very first one was Eh, what's up, Doc? And, gee, it floored [the audience]!

They expected the rabbit to scream, or anything but make a casual remark--here's a guy with a gun in his face! It got such a laugh that we said, 'Boy, we'll do that every chance we get.' It became a series of 'What's up, Docs?'.

He was the most timid of animals, yet he had courage and brashness. The whole gimmick was a rabbit so cocky that he wasn't afraid of a guy with a gun who was hunting him.

The phrase "What's up, doc?" was used at his High School.

Remembering the ‘What’s up, Doc?’ expression from his high-school days in Texas, Avery had decided to place it in the mouth of a sharp Brooklynese rabbit who knew everything.

The first time Bugs uses the phrase is in the cartoon Wild Hare 1940. The rabbit walks up to Elmer Fudd who is hunting for him with a large gun and casually asks, “What’s up Doc?”

It's their original phrase. May be inspired from What's up Phrase which appears previously

The phrase appears in Jack London's The Sea Wolf (1904), chapter 25 (-- "What's up?" I asked Wolf Larson.--)

Another referance to 'whatsup' can be found in a short story The Adventures of Shamrock Jolnes from the collection Sixes and Sevens (1911) by acclaimed American short story writer O' Henry (1862-1910). The character Shamrock Jolnes says, "Good morning, Whatsup."