…
And I’m totally stymied for a response. As some of you may know, I’m not 'merkin. But I’ve recently gotten quite a few of them on my IM service, and they invariably seem to open with “Hey, what’s up!”
And what’s the expected response? Help? 
The answer is always, “not much”. Of course, then you go on and describe your recent operation, marriage, death in the family, Nobel Prize, etc.
What’s up = what’s going on with you = what are you up to = what’s happening, all of which are more or less a variation of “Hello, how are you?”
Responses? “Doin’ okay,” “chillin’,” or any reasonable response to a greeting.
“Nothing, what’s up with you?”
“Oh, [insert description of recent event here]. What are you up to?”
“Hay”
“dude sup”
Actually, the correct response to this (at least when asked in passing by a co-worker) is “Good, you?”, followed three steps later by the realization of what you just said and some appropriate forehead slapping.
EXACTLY!!!
Actually, I usually answer “fine”, followed by the 3-step/head slap.
“I thought you knew?!”
“The sky.”
My sister and I came up with that when we were really little. It’s a habit now.
“Both breasts and my attitude.”
I think the question isn’t really looking for an answer to “what’s up”, it’s just a greeting. I always respond with “hey”.
I know, it’s eloquent.
I think the question isn’t really looking for an answer to “what’s up”, it’s just a greeting. I always respond with “hey”. That way, the conversation volleys back to the other person and it’s their job to come up with something to continue it, not mine.
I know, it’s eloquent.
Sorry about that… not sure how it happened.
Bah! “What’s up?” always used to confuse me. Here in the U.K. it is more likely to carry a meaning of “what is wrong?” and I was always muddled when 'phoning a friend (originally from N.Y.C.) and she said “what’s up?” (especially as I had usally 'phoned to let her let off steam about a variety of bad stuff. I got the idea eventually, although I stil don’t like it much.
It’s analogous to the Spanish “que pasa?” and the German “wie geht’s?”.
Maybe we Dopers can spearhead a movement to revive the old “how’s tricks?”…?
So I guess the correct response to “What’s up?” should be “What’s up” 
Most people I know just reply with “Not much”.
My grandmother always says, “My blood pressure.”
Yo.
I always reply, “Not much,” no matter what is actually happening. Then later in the conversation, you talk about what is actually happening. The only exception is in cases of personal crisis, when I will say, “Nothing good,” and then elaborate if the other person really wants to know.