An elaborate fabrication. This phrase uses pack in the sense of a large collection of abstract objects, and, as it has been since Shakespeare’s time, in a disparaging sense. Thomas Jefferson used it in a letter of 1763: “Would you rather that I should write you a pack of lies?” A closely related expression is a tissue of lies. It uses tissue in the figurative sense of a woven fabric—that is, an intricate intertwining of separate elements, similarly abstract.