How's it hangin' is a very informal way to say hello to your fellow. What is actually hanging, is evident from the following example.
- Hi, Mike, how's it hangin'?
- A little bit to the left.
by KAO1 November 2, 2005
How's it hangin is a polite greeting from one man to another. "It", in the context, is the man's penis. Presumably if it's hanging well, the man is doing well.
Jerry: Hey John, how's it hangin?
John: Long and low, Jerry, long and low!
Jerry: that's great!
by RexGibson April 26, 2004