Cus is just a shortened version of the word BECAUSE. It's often used in song writing/singing, simply for the fact that it flows better and is much easier to sing.
"Cus well never have to fight again there's no reason now
cus all you need is love and love is all you've ever found don't you know my friend that you know how
cus i am you and you are everything you think about" ~ Corey Gray