Apparently even Bo has no idea
5. What made you change your name from Mary Cathleen - with the nickname Cathy - to Bo?
Derek: I was 17, living in Europe and I just wanted something more grown-up, I guess, than Cathy. ... I tried many, many names and I really don't know where Bo came from. But it is unusual and I liked it right away. And I never had a problem adjusting to it.
(maybe from Bim-bo... ;) )