A rebroadcast of a British radio quiz show on National Public Radio here gave an etymology of “swashbuckler” as deriving from tapping or beating one’s sword (swashing) against one’s shield (buckler).  It sounds plausible to me but is it correct?

Yes.  OED:

[f. SWASH v. + BUCKLER n.; hence lit. one who makes a noise by striking his own or his opponent’s shield with his sword.]

Of course one doesn’t strike one’s opponent’s shield with one’s sword as a gesture. The modern sense of a swashbuckler brings visions of a dashing movie character with a ready smile ala Errol Flynn. I’m sure the original swashbucklers were someone you’d rather not meet.

However, if you’re considering a snazzy career move, try this quiz first:

http://www.quizilla.com/users/Dawg/quizzes/What%20Type%20Of%20Swashbuckler%20Are%20You

(Be forewarned, the results probably won’t show up. That may be the answer to the quiz: real swashbucklers don’t take quizzes.)