I had an uncle who told me that "pilchards" was a swearword. I heard a presenter use it on a programme about fishing when I was about 6, and ran to tell my mum.
Sometimes I still catch myself using it..
A childhood belief about swearing

others believed things like this
- I used to think that masterbating was a fishing term for people who fish really wellAnon
- Me & my brother used to think that the science programme QED was called "KWED" rather then "KYOO EE DEE".— Richard Davies
- my uncle was a doctor, so when he told me his vw could fly, i believed.— boodles
- Up until I was nine my uncle told me that the round bales of hay were cow eggs.— Fred
- My uncle told me if you twist your bellybutton, your bum will fall off, I was terrified!— little2ft
- When I was little my uncle had a huge adams apple. He told me he'd swallowed a marble.— Stacey