It's the opposite of a "busy bee". It is said that the term originates from the story of an Indian boy who is residing in England during Christmas. While all the locals are busy celebrating Christmas, the boy has nothing on his calendar and is available to meet anyone at this time. He feels alienated and hence the term "available alien".

Note that unlike the term "available ass", there is no sexual connotation here.
Katy: Hey Dhruv, are you available on the 22nd of December to meet? That is the only day I have free on my diary?
Dhruv: Yeah, I can meet anytime. I am totally an available alien.
by dhruvc November 20, 2016
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