With it - joint (3)
I believe the answer is:
hip
I believe this is a double definition.
'with it' is the first definition.
(both can mean cool or fashionable)
'joint' is the second definition.
(part of body)
(Other definitions for hip that I've seen before include "Pelvis" , "Fruit of the dog-rose" , "Slangily stylish like rose fruit" , "Following trends" , "Fashionable" .)