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.