Something Heard For The First Time, News
noun
The first time something reaches your ear, it's something heard for the first time. If it's never entered your ear before, it's news to you!
はつみみ
This is a jukugo word that uses kun'yomi instead of on'yomi readings, because 初 behaves more like a prefix here. You already know how to read 耳, but you haven't learned the kun'yomi for 初 yet, so here's a mnemonic to help you remember it: Whatever it is, this something heard for the first time, be sure to hold onto your hats (はつ)! It's going to be something big, and I'm pretty sure it's going to be news to me, too.
ええっ、それは初耳です。
Huh, that's the first I've heard of it.
マジ？そんな話、初耳だぞ！？
Seriously? That's all news to me!
この言葉は、初耳の人も多いかもしれませんね。
This term may be new to many of you.