The Collaborative International Dictionary of English v.0.48:

Lung \Lung\ (l[u^]ng), n. [OE. lunge, AS. lunge, pl. lungen;
   akin to D. long, G. lunge, Icel. & Sw. lunga, Dan. lunge, all
   prob. from the root of E. light. [root]125. See Light not
   heavy.] (Anat.)
   An organ for a["e]rial respiration; -- commonly in the
   plural.
   [1913 Webster]

         My lungs began to crow
         like chanticleer.                        --Shak.
   [1913 Webster]

   Note: In all air-breathing vertebrates the lungs are
         developed from the ventral wall of the esophagus as a
         pouch which divides into two sacs. In amphibians and
         many reptiles the lungs retain very nearly this
         primitive saclike character, but in the higher forms
         the connection with the esophagus becomes elongated
         into the windpipe and the inner walls of the sacs
         become more and more divided, until, in the mammals,
         the air spaces become minutely divided into tubes
         ending in small air cells, in the walls of which the
         blood circulates in a fine network of capillaries. In
         mammals the lungs are more or less divided into lobes,
         and each lung occupies a separate cavity in the thorax.
         See Respiration.
         [1913 Webster]

   Lung fever (Med.), pneumonia.

   Lung flower (Bot.), a species of gentian (Gentian
      Pneumonanthe).

   Lung lichen (Bot.), tree lungwort. See under Lungwort.

   Lung sac (Zool.), one of the breathing organs of spiders
      and snails.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lung
    n 1: either of two saclike respiratory organs in the chest of
         vertebrates; serves to remove carbon dioxide and provide
         oxygen to the blood

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "lung":
   abdomen, anus, appendix, bellows, blind gut, bowels, brain, cecum,
   colon, ctenidia, duodenum, endocardium, entrails, foregut, giblets,
   gills, gizzard, guts, heart, hindgut, innards, inner mechanism,
   insides, internals, intestine, inwards, jejunum, kidney, kishkes,
   large intestine, lights, liver, liver and lights, lungs, midgut,
   perineum, pump, pylorus, rectum, small intestine, spleen, stomach,
   ticker, tripes, vermiform appendix, viscera, vitals, works