I think that Stallybrass is saying that no thoughts are original. All thoughts originate from a collective force or consciousness, in much the way that words do. You are not the originator of the words you say; you are merely borrowing them from a “linguistic warehouse”, of sorts, in an effort to communicate with others.
In the same way, Stallybrass is saying that you borrow thoughts to communicate ideas. However, with thoughts the media in which you express these “borrowed” thoughts are much more varried than that of words. For instance, in a research paper, a medium of written form in this case, you borrow thoughts from various sources and compile them into an organized paper for the purpose of communicating these thoughts themselves to other people.