The Collaborative International Dictionary of English v.0.48:

Backlog \Back"log`\ (b[a^]k"l[o^]g`; 115), n. [Back, a. + log.]
   1. A large stick of wood, forming the back of a fire on the
      hearth. Contrasted to forestick. [U.S.]
      [1913 Webster]

            There was first a backlog, from fifteen to four and
            twenty inches in diameter and five feet long,
            imbedded in the ashes.                --S. G.
                                                  Goodrich.
      [1913 Webster]

   2. An accumulation (of tasks or of materials) that were not
      performed or processed at the normal time and require
      attention; as, unexpected demand led to a large backlog of
      unfilled orders; politically motivated delays led to a
      large backlog of unconfirmed judicial appointments.
      [PJC]

WordNet (r) 3.0 (2006):

backlog
    n 1: an accumulation of jobs not done or materials not processed
         that are yet to be dealt with (especially unfilled customer
         orders for products or services)
    2: the large log at the back of a hearth fire
    3: something kept back or saved for future use or a special
       purpose [syn: reserve, backlog, stockpile]
    v 1: accumulate and create a backlog

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "backlog":
   abundance, accumulation, amassment, bavin, brush, brushwood,
   budget, cache, collection, commissariat, commissary, cornucopia,
   cumulation, dump, fagot, firewood, heap, hoard, inventory,
   kindling, kindlings, larder, log, mass, material, materials,
   materiel, munitions, nest egg, pile, plenitude, plenty,
   provisionment, provisions, rations, repertoire, repertory, reserve,
   reserve fund, reserve supply, reserves, reservoir, resource, rick,
   savings, sinking fund, something in reserve, stack, stock,
   stock-in-trade, stockpile, store, stores, stovewood, supplies,
   supply on hand, treasure, treasury, unexpended balance, wood,
   yule clog, yule log