When I was very young, I apparently had a dream where my mom told me that penises were called "uddies" - some sort of bizarre diminutive form of "udder". It made sense to me at the time. I mean, I was pretty familiar with cows, living in Wisconsin as I did, and a penis seemed pretty similar to a cow's teat to ME. Pink, squirts liquid... I believed this for AGES because, well, it made a twisted sort of sense.
Then my mom found out and laughed at me, and insisted that I must have dreamed her telling me. I was really mad at her for a while, because I was SURE she had told me that.
A childhood belief about rude bits

others believed things like this
- when i was 2 or 3 believed that i had lots of animals in my tummy it made sense my mum had baby in hers.— kjc
- i used to believe that the "fast lane" actually had some sort of tracks on it that made you go faster.— erin
- I used to think that the priest at church was JESUS.
He had a beard.... it made sense— Joe B - I used to believe that broccoli grew on trees. It looks like a tree, so it made sense, right?— Catherine...
- I used to think that Batman was Blackman. He wore a black superhero outfit, so it made sense to me.Anon
- I believed when young that horses with white faces were sick. My dad a great sense of humour— John Cavanagh