Something heard for the first time; a piece of information that is new or surprising to the listener.
それは初耳だよ。どこで知ったの？
That's the first I'm hearing of that. Where did you hear about it?
「彼、結婚するらしいよ」—「え、初耳！」
"I heard he's getting married." — "What? That's news to me!"
その制度の変更は私にとって初耳だったので、詳しい説明を求めた。
The change in the system was new to me, so I asked for a detailed explanation.