"Doo Dah", like "Thingummajig" etc., is just a term for a thing you can't describe or can't be bothered to. Its application to the male genitalia is basically as an alternative to calling it your "thing".

The word "thing" to mean something essentially non-decribed or non-describable in English is quite old and probably originates from the Danish. Formal communal meetings were known as "The Thing" in Danelaw areas.

;-)
Ian