This is made up of two words:
Chill and Relaxing.
I started using this term when I heard a friend's mom say-"Just chill and relax" really fast. when someone asks you-"Supsup" and you want to say-"Just chillin' and relaxin'"
Then, you should say-"chill-axn'" or just-"chillaxn"
Person 1: Supatchu?
Person 2: oh juss chill-axn. You?
Person 1: Samesame, man.