English lover, The first time Bugs uses the phrase is in the cartoon Wild Hare 1941. The rabbit walks up to Elmer Fudd who is hunting for him with a large gun and casually asks, “What’s up Doc?” What’s up? is slang for What’s going on? or What’s happening? Doc is a short form of Doctor. It has the slang meaning of ‘the person who is in charge of a situation’…or ‘the person who makes the decisions.” Another form of this use is “You’re the doctor,” meaning “ You are the expert; it is for you to decide.” Bugs is of course being sarcastic when he asks the hunter “What’s up Doc?” He easily outwits the silly Elmer Fudd and escapes. The rabbit is always in control of the situation in his cartoons and “What’s up Doc?” has become a very famous phrase thanks to Bugs Bunny.

Glyn answered the question correctly but also "doc" is short for doctor. He doesn't mean an actual doctor. He is only being funny. What's up doc = How are you doing today, doctor? (but it's not funny if you say it like this)

It means what is going on. What is happying