The Collaborative International Dictionary of English v.0.48:

gasbag \gasbag\ n.
   1. a person who talks a great deal about uninteresting
      topics.

   Syn: windbag.
        [WordNet 1.5]

   2. the bag containing the gas in a balloon.

   Syn: envelope.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

gasbag
    n 1: a boring person who talks a great deal about uninteresting
         topics [syn: gasbag, windbag]
    2: the bag containing the gas in a balloon [syn: envelope,
       gasbag]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "gasbag":
   Captain Bobadil, Gascon, Graf Zeppelin, Texan, aerostat,
   agreeable rattle, airship, babbler, ballonet, balloon, big mouth,
   big talker, blab, blabberer, blatherer, blimp, blower, blowhard,
   blusterer, boaster, brag, braggadocio, braggart, chatterbox,
   chatterer, dirigible, dirigible balloon, fanfaron, gabber, gabbler,
   gasconader, gibble-gabbler, great talker, hector, hot-air artist,
   idle chatterer, jabberer, jay, lighter-than-air craft, magpie,
   miles gloriosus, moulin a paroles, patterer, prater, prattler,
   rattle, rigid airship, rodomontade, semirigid airship, ship,
   windbag, windjammer, windy, word-slinger, zeppelin