This happened to my brother. He used to call everyone "maggot" as a rude term to get a rise out of people. It was many years later he found out that when he thought he was calling people a racist term, he was really calling them a form of larvae.
A childhood belief about speaking

others believed things like this
- I thought the term maiden name actually applied to women who were maidens.— Natasha
- I used to think that the initials NYC stood for "nobody you're calling" Anon
- From the Carpenters hit I always sang "calling octopus of interplanetary craft" instead of "occupants".— Paul
- I used to believe that the term "being mugged" meant someone beat someone else with a coffee mug.— Matt Thiel
- i used to think that the term 'necking' referred to boyfriends and girlfriends who rubbed their necks togetherAnon
- I used to believe that mosquito larvae were baby hammerhead sharks. Made the ditch a very dangerous place!— Karen