I thought that "Oh, the humanity!" was an exclamation meant for pointing out that something was very humanlike.
I used to think Yiddish was pronounced to rhyme with Swedish.
I believed that the word optimistic was something to do with opticians and being happy with your eyesight.
When I was little in the early 60's we lived in an area that was nearly all white.Back then the Politically Correct thing to call black people was "Negros".The first time I heard that term I got a weird image of a person who had knees growing all over them ( as in "knee grows").I thought it was a weird disability of some sort.
Not me but my classmate. He thought that stereotyping was listening to the stereo.
When people used the word "stationary" to mean staying still, I thought they were talking about the stuff you write with.
i used to think the New York accent was the accent all old people had because as a child all the old people I knew (grandparents, great grandma, great aunts, uncles, etc) had thick NY accents. Also that when you become old you automatically develop a New York accent as like a rite of passage ...I was a strange child
I thought "incessantly" meant "often and annoyingly".
Everybody described the conscience as a "little voice in your head", so I thought that *all* thoughts were from the conscience-- essentially, I mixed the conscience up with the internal monologue.
I had (and still have) a lot of random thoughts, and when I was a kid, I was often told that I was very well-mannered for my age, so I thought that because I was so polite, my conscience was "bored" and just said random stuff.
I used to think that clinical depression was called "cynical" depression.
I thought "unrest" was the opposite of rest. And, since I hated resting, I thought it was a good thing.
I also thought "uneasy" meant "hard" or "difficult".
I forgot the expression "not cricket", so I said, "not tennis" instead.
I mixed "confidence" up with "competence" so when I was watching an episode of a kids' show that involved the idea of "losing your confidence", I was like, "But she didn't lose her confidence! She's still good at it, just too nervous to actually do it."
When I was little I thought it was literally possible to "waste your breath". Like, you could actually talk too much and would not have any air left for speaking, and you'd not be able to talk again for the rest of your life.
I had these pajamas that were too loose for me and my mother called them "loosies" but I thought she was saying "Lucy's" and the reason they were too big was because they were meant for someone named Lucy, not me.
I thought that "losing your cool" meant you weren't cool anymore (as in weren't awesome) so I felt offended when my mother told me I'd lost my cool.
I thought that people only said, "Guess what?!" when delivering good news. So when Dad said, "Hey, guess what?! Eva broke her arm!!" (Eva is my friend and she's OK now, this was years ago), I was shocked at him and wondered why he thought it was good news that my friend broke her arm.
I used to believe that the word "artificial" was actually "art official," like artifical flavored candy was some sort of official art or something.
For a while I believed that saliva was a naughty word to say, because it seemed like a private bodily fluid that you should not mention in public.
I thought that when people talked about skyscrapers they were referring to the jet airliners that I had always seen flying overhead leaving behind them white scrape marks on the sky. My brother corrected me one day when I pointed up and said, "Look, there goes a skyscraper." After he told me that it really meant a super tall building I thought that was absurd, because a tall building doesn't leave a scrape mark!