A rigadoon is a lively 17th/18th century dance or the music for it. Maybe the little beggerman is making a little mouth music. I haven't looked at the DT version, but I know sometimes he's singing something that sounds pretty much like nonsense syllables (skina marink adoodle - Don't quote me on the spelling, I've not seen it in print). That seems like a stretch to me. I'd be interested to hear from someone who knows.