The Collaborative International Dictionary of English v.0.48:

Sponge \Sponge\ (sp[u^]nj), n. [OF. esponge, F. ['e]ponge, L.
   spongia, Gr. spoggia`, spo`ggos. Cf. Fungus, Spunk.]
   [Formerly written also spunge.]
   1. (Zool.) Any one of numerous species of Spongiae, or
      Porifera. See Illust. and Note under Spongiae.
      [1913 Webster]

   2. The elastic fibrous skeleton of many species of horny
      Spongiae (Keratosa), used for many purposes, especially
      the varieties of the genus Spongia. The most valuable
      sponges are found in the Mediterranean and the Red Sea,
      and on the coasts of Florida and the West Indies.
      [1913 Webster]

   3. Fig.: One who lives upon others; a pertinacious and
      indolent dependent; a parasite; a sponger.
      [1913 Webster]

   4. Any spongelike substance. Specifically:
      (a) Dough before it is kneaded and formed into loaves, and
          after it is converted into a light, spongy mass by the
          agency of the yeast or leaven.
      (b) Iron from the puddling furnace, in a pasty condition.
      (c) Iron ore, in masses, reduced but not melted or worked.
          [1913 Webster]

   5. (Gun.) A mop for cleaning the bore of a cannon after a
      discharge. It consists of a cylinder of wood, covered with
      sheepskin with the wool on, or cloth with a heavy looped
      nap, and having a handle, or staff.
      [1913 Webster]

   6. (Far.) The extremity, or point, of a horseshoe, answering
      to the heel.
      [1913 Webster]

   Bath sponge, any one of several varieties of coarse
      commercial sponges, especially Spongia equina.

   Cup sponge, a toilet sponge growing in a cup-shaped form.


   Glass sponge. See Glass-sponge, in the Vocabulary.

   Glove sponge, a variety of commercial sponge (Spongia
      officinalis, variety tubulifera), having very fine
      fibers, native of Florida, and the West Indies.

   Grass sponge, any one of several varieties of coarse
      commercial sponges having the surface irregularly tufted,
      as Spongia graminea, and Spongia equina, variety
      cerebriformis, of Florida and the West Indies.

   Horse sponge, a coarse commercial sponge, especially
      Spongia equina.

   Platinum sponge. (Chem.) See under Platinum.

   Pyrotechnical sponge, a substance made of mushrooms or
      fungi, which are boiled in water, dried, and beaten, then
      put in a strong lye prepared with saltpeter, and again
      dried in an oven. This makes the black match, or tinder,
      brought from Germany.

   Sheep's-wool sponge, a fine and durable commercial sponge
      (Spongia equina, variety gossypina) found in Florida and
      the West Indies. The surface is covered with larger and
      smaller tufts, having the oscula between them.

   Sponge cake, a kind of sweet cake which is light and
      spongy.

   Sponge lead, or Spongy lead (Chem.), metallic lead
      brought to a spongy form by reduction of lead salts, or by
      compressing finely divided lead; -- used in secondary
      batteries and otherwise.

   Sponge tree (Bot.), a tropical leguminous tree (Acacia
      Farnesiana), with deliciously fragrant flowers, which are
      used in perfumery.

   Toilet sponge, a very fine and superior variety of
      Mediterranean sponge (Spongia officinalis, variety
      Mediterranea); -- called also Turkish sponge.

   To set a sponge (Cookery), to leaven a small mass of flour,
      to be used in leavening a larger quantity.

   To throw up the sponge, to give up a contest; to
      acknowledge defeat; -- from a custom of the prize ring,
      the person employed to sponge a pugilist between rounds
      throwing his sponge in the air in token of defeat; -- now,
      throw in the towel is more common, and has the same
      origin and meaning. [Cant or Slang] "He was too brave a
      man to throw up the sponge to fate." --Lowell.

   Vegetable sponge. (Bot.) See Loof.

   Velvet sponge, a fine, soft commercial sponge (Spongia
      equina, variety meandriniformis) found in Florida and the
      West Indies.

   Vitreous sponge. See Glass-sponge.

   Yellow sponge, a common and valuable commercial sponge
      (Spongia agaricina, variety corlosia) found in Florida
      and the West Indies.
      [1913 Webster]

WordNet (r) 3.0 (2006):

throw in the towel
    v 1: give up in the face of defeat of lacking hope; admit
         defeat; "In the second round, the challenger gave up" [syn:
         drop out, give up, fall by the wayside, drop by the
         wayside, throw in, throw in the towel, quit, chuck
         up the sponge] [ant: enter, participate]