Moby ThesaurusCaptain Bobadil, Gascon, Texan, agreeable rattle, babbler, big mouth, big talker, blab, blabberer, blatherer, blower, blowhard, blusterer, boaster, brag, braggadocio, braggart, chatterbox, chatterer, fanfaron, gabber, gabbler, gasbag, gasconader, gibble-gabbler, great talker, hector, hot-air artist, idle chatterer, jabberer, jay, magpie, miles gloriosus, moulin a paroles, patterer, prater, prattler, rattle, windjammer, windy, word-slinger
A windbag is a person who talks too much.
Welsh WindbagWelsh Windbag is a derogatory term for a Welsh person, used mostly by English people. Neil Kinnock was nicknamed the "Welsh Windbag", although it had been applied to many Welsh people before him, for instance Dylan Thomas, Aneurin Bevan, and Kenneth Griffith.
It should be noted that the term "Welsh Wizard" has been used when being complimentary about a welshman. This was a nickname for Lloyd George, a UK Prime Minister in the early 20th century. It has been used for others, such as when footballer Ryan Giggs scores a memorable goal.
Given the above, its origin seems simply to be alliterative, with a positive or negative word beginning with W being selected dependent on the speaker's intent.