Play it by.....WHAT??

I learned something tonight that I have been saying and hearing wrong for the last 26 years. I have always thought the saying was "Let's play it by year". I don't know why it was year because that doesn't makes sense but that is how I have been saying my entire existence. I corrected a friend tonight when they said "Let's play it by ear" and then after asking multiple people which way was correct, I figured out I had been wrong all along. How did I never know this? It's funny if you ask me!