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.

A man who messes around with other men, but no one would ever know by looking or talking to him. Used by gay black men to identify masculine gay men or DL Brothas.

becoming relaxed by chilling with your friends

hanging out with your homies

It means your relaxing at the same time your chillin.

to chill (to cool down) and relax at your pace