A ridiculous term, basically a tautology, which has absolutely no reason for existing other than the middle consonant of "relax" is the same as the last consonant of "chill", and both words have the same meaning. A waste of everyone's time. It's just not cool.
"I'm too cool to say "relax" or "chill" so I have to combine them and say "chillax", when actually the two words are synonyms anyway. Aren't I clever eh?"
"Well you would have been quite clever, but it's been said a million times before and now has over 40 entries on the Urban Dictionary, so sorry love, I'm gonna have to overrule you on that. "Not three bad" was funny once, you know?"