bonnie star

A way of saying penis in mixed company without anyone really knowing what's being said especially children, used in the song The Scotsman

Bonnie means beautiful in Scottish, or a beautiful woman

Star can mean a star in the sky like in English, or something grassy in a bog

Maybe the composer of the song was being creative describing where the event took place STAR, and the one performing the event BONNIE

Around the Bonnie star the Scott's Kilt did lift and show