Chillaxing means when your sitting doing nothing but relaxing after a long day , either because your mad/exhausted/angry
Girlfriend : babe why your not replying
Boyfriend : I'm just chillaxing
Girlfriend: oh babe !! Tell me what's wrong ? Do you want me to come over ?
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Girlfriend : babe why your not replying ?
Boyfriend: I'll talk to you later when I calm down !
Girlfriend : oh really ! Don't even bother yourself