As a child, my mother would always call out "banzai" when she wanted me to raise my arms above my head so she could put on or take off a pullover shirt. I assumed that banzai was the word for "stick your hands in the air!"
It wasn't until well into my adult life that my mother explained to me that, no, banzai does not mean "stick your hands in the air." It's a Japanese word meaning "ten thousand years", shouted as a term of approbation and accompanied by (you guessed it) throwing one's hands into the air. My mother was using it as a play term; in the United States, when you get your child to throw his hands into the air, you might accompany it with a shout of "Touchdown!"
What words have you gotten wrong your entire life?
Related: I Used to Believe.
Reminder: This upcoming Sunday is Mother's Day in the United States, and it honors all mothers, not just your own.
Bonus niece chatter: For a time, one of my nieces called a sneeze a bleshoo.