Green’s Dictionary of Slang

windbag n.

1. [mid-19C+] a braggart, a boaster, a ‘loudmouth’; thus as v., to boast.

2. [mid-18C; 1900s–40s] (US black, also windpump) in pl., the lungs [their role in the body].

3. [1930s] (UK Und.) a confidence trick, based on selling envelopes that are apparently filled with valuables; in the event they contain only rubbish; also attrib.