A do-do usually refers to an
immature penis, but it can and is used to also, as a pejorative, to identify or describe the shrinking or always very small
penis of an older or elderly white man.
Come on, Bobby, stand next to the toilet like Daddy and
aim your do-do, right in the middle, try to hit the toilet
paper.
I am sorry honey, Grandma is too
tired to play with Grandpa's do-do, you play with it.