When I was about 11, I heard or saw the phrase "jerk off" somewhere. I didn't know what it really meant, so I deducted that it meant to fool around or goof off. So my older cousin took me to the mall one day, and I at the time hated shopping, so I said to her, loudly, "Let's go jerk off in the arcade!!"
Yeah, it was pretty shortly that I learned what "jerk off" really meant. I cringe now thinking of how many times I said that without knowing that I was suggested that me and my friends go masturbate.
A childhood belief about speaking

others believed things like this
- I thought the phrase "At one's disposal" meant you should dispose of the person or thing.— Geirskogul
- I used to think that an imbecile was some type of seal, before knowing that it meant someone stupid.Anon
- i used to believe that sex meant love and that having sex meant the two people would live happily ever after.— amy
- I used to think a second cousin meant a cousin’s cousin.— Carlos Almonacid
- i used to think the word "friendship" meant a special kind of rocketship that you went on with your friends.— chips ahoy!
- When I first heard the words "flea market", I actually thought it meant "selling fleas".Anon