Noun

1. A bag or bladder to hold a reservoir of gas, as in a hot-air balloon.
   • 1914: Eric Henry Stuart Bruce, Aircraft in War http://books.google.com/books?vid=OCLC01239540&id=VjxAUINOShwC&pg=PA24&lpg=PA24&dq=gasbag&as_brr=1

     By this method of construction a considerable amount of support can be imparted to the gasbag, though it does not dispense with the services of the ballonet, as does the entirely rigid type.

2. Metaphorically, a person who is overly garrulous or prone to making empty, unsupportable statements.
   • 1911: Edward Everett Hale, Dramatists of To-day: Rostand, Hauptmann, Sudermann, Pinero, Shaw, Phillips, Maeterlinck http://books.google.com/books?vid=OCLC01593656&id=cWByH3gSTjUC&pg=PA124&lpg=PA124&dq=gasbag&as_brr=1

     Not that the Reverend James is absolutely a pretentious gasbag any more than Marchbanks is an inspired prophet. He has a definite, a positive part in the world's work.