Someone, X, is living at home, and begins spending nights away, staying at a mate's flat. He takes his CDs ad DVDs and has them at his mates. Over the weeks, he leaves more and more clothes there as he takes off dirty clothes and leaves them, and puts on clean clothes he brought with him for work the next day. The speaker is perhaps using hyperbole, exaggerating for effect, and commenting that from the amount of things of his still remaining in his room in the parental home, it's as if he has moved out = or, very nearly = 'as good as' moved out. "He's as good as moved out!"
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