Ring-Dang-Doo is a nonsense slang word typically used to describe a women's genitals similar to "Pussy".
The word has no etymology and first appeared in a song by Sam the Sham and the Pharaohs but the word also made an appearance in the 2018 western action-adventure game Red Dead Redemption 2 in a campfire song.
Ring-Dang-Doo now what is that? It's soft and round like a pussycat, Got a hole in the middle and it's split in two; And that's what you call the Ring-Dang-Doo.