It comes from the Alpha male expression,
Alpha males are the individuals in the community with the highest social rank. In other words the guy takes the girl.
As awkward it may seem they are usually the douches who treat everyone like crap. But by doing so they show that they have confidence in themselves, a very rare
artifact now a days, thus having something what everyone would appreciate they earn respect.
The Alpha as fuck expression is usually used after you did or you are doing something what would consider you as a douche, but you don't care because you know
the golden rule i described above.
But lately people started using it everywhere and on everything.