My sister used to think that "spit" was a bad word. She meant "shit".
A childhood belief about swearing

others believed things like this
- my husband believed, until recently, that the word "several" meant seven of something. — sheryl
- I used to believe that "nuptial" was a naughty word. I thought it meant nipple. XD— Sara Hagen
- I used to believe that your arse cheeks were actually full of shit!Anon
- i used to think the word "friendship" meant a special kind of rocketship that you went on with your friends.— chips ahoy!
- For years I heard the word "pedophile" and thought it meant a pedicurist's tool.— Lauren
- I fully believed, up until about 6 years old, that the word 'accident' was a noun that meant 'burnt toast'.— Karl Goetz