I used to believe that accord was short for accordion
I used to think that the word pizzazz was something to do with pizza.
I used to think gladiators were meant to be some sort of alligator.
I used to think Art Deco was Aunt Deco, and wondered who this famously artistic woman was meant to be.
When I was little I used to believe that if you told a lie, your nose would grow in length.
Because of Beauty and the Beast, I thought that "provincial" meant "boring" because she sings, "I want much more than this provincial life!".
It also made me think "primeval" meant "prime evil", e.g really bad, because Belle uses that word to describe Gaston.
At 31 I moved from Italy to London, UK. I used to believe that the phrase "out of spit" was a particularly graphic and rather dramatic way of expressing one's animosity and vengeful intententions. Only 8 years later, after moving back to Italy, I eventually realized that one is supposed to say "out of spite".
There's a website called TV Tropes that has a trope called "Heroic BSoD", which means a hero has an emotional breakdown.
Because of this, I thought BSoD was a disorder, like OCD or PTSD, that meant you were prone to emotional breakdowns.
Later, I learnt that it's actually an abbreviation for Blue Screen of Death.
I thought for a long time that the word "awry" was pronounced like "aw-ree" (rhymes with sorry) , instead of "uh-rye". The weird part? I had heard people say it lots of times, but it took ages before I realized it was the same word
I used to think you could go to jail just by saying a word wrong during a speech.
I used to think that weasel was short for wooden easel, and was confused as to why someone would be called a piece of art equipment as an insult.
I thought "prison" and "prism" are the same words as each other.
I used to think that retarded was the same word as retired.
I used to think that if something was out of the question, then you were banned from asking for it.
When I was 15, I accidentally referred to my baby cousin's Christening as his funeral. Well, at least I found it funny myself.
I used to think headquarters meant somebody's head cut into 4.
I thought the phrase "throw the towel in" had to do with throwing one's swimming towel into the pool/ocean/lake/river.
Stereotypically, I used to think that all Indians used the phrase 'oh my God' in just about every sentence.
Until one of my parents actually used the word out loud, I used to see the word 'deluxe' on my pet rescue game, and thought it was pronounced 'DELCKS-chur'.
I thought that polystyrene was pronounced 'polly-STEE-LEE-an'.