I consider "ding-dong" a reasonably common euphemism for "penis," or at least it was when I was young; maybe it's not any more. I've always assumed (without evidence) that it was shortened to the probably more common "dong."
Words that sound a whole lot like ding-dong have regularly been used as jokey euphemisms for penises. An example that comes to mind is that very annoying but catchy Chuck Berry song from the 1970s called "My Ding-a-ling." The entire song consists of lyrics that sound as though they might be about bells but might be about...something else.
(And if I find myself humming this song the rest of the day, I'm going to be very, very annoyed with my brain.)