'wah gwan' means 'whats going on', taken from the carribean accent 'patois' and daily used as UK slang talk for hello, spelt as 'Wag1' on chat sites and MSN as it takes less time to type
(i)person 1: wag1
person 2: nothing much
(ii)person 1: im mad
person 2: why wah gwan?
use it in any other way and your using it wrong!