Dude, just chillax, just because it's not your birthday doesn't mean that you won't have fun in the monkey jamboree. I'm out like a decent hurricane, schlater.
It's a word put together from Chillin' and Relaxing. It actually means the same. If somebody should cool down you could say he or she has to relax or has to chill out. If you decide not to choose between these to, or you haven't made a decision while you already have to say the word, you say 'Chillax'.
It could also mean having a good time with your friends.
When one is experiencing extremely high levels of various emotions that may agitate another person in their company and really needs to just chill out or relax, the annoyed party in their presence can tell them to take a chillaxative, therefore rendering it appropriate for that person to fully let go and purge their stresses until they reach a serene state of calmness.
