'How you hanging?'

what does 'How you hanging?' mean?

It's a casual (VERY casual) greeting spoken by one man to another. Another form is "How's it hanging?" The literal reference is to the male organ, but the question just means "How are you?" or "How's everything?" To stay out of trouble, do not use this line except with very close male friends.

"Usually considered vulgar" as they say.

The polite version of this is still used by bespoke tailors when measuring a man for a suit - 'How do you dress, Sir?' - which side do you prefer to place your private parts. Presumably the alloted side is then made a little bigger.

...or a lot ;-) Suits you, sir!

Not an expression to be used with casual acquaintances, unless perhaps you frequent Hampstead Heath(?) - there is no standard response these days, although I recall that there might have been a reply such as "like a monkey"