It's the combinatioin of the words 'chill' and 'relax'.
Hey buddy I screwed up
oh common! just chillax
To light the joint after a hard day of adulting
Couldn't wait to get home after work to chillax
To relax and chill at the same time; usually stated/said by someone who is cool and currently chillaxing at the moment
Jacob needs to chillax because he got a stick in his ass
It is when reach such a being of chilling out and relaxing that you enter another realm of laziness.