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.
Woo man, don't get so angry, chillax a little!

or

Hangin' out with my homies today was super chillaxed.
by Jasper Vondeling August 27, 2007
It means your relaxing at the same time your chillin.
I'm Chillaxed at the beach!
by Pimpilishous March 26, 2009
a state of ultimate chill and relaxation.

See chill, relax
I was at home chillaxin' like a fiend
by Chris da Homie January 11, 2004
A mixture between the terms "chill out" and "relax".
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.
by El_Scorcho September 4, 2003
someting to help you chill,relax e.g. hashish
you need to take a chillaxative man
by pyewhacker October 28, 2005

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.

Stop freaking out, you really need to take a chillaxative!
by Angelina Guglielmo March 17, 2008