a bolt from the blue; a sudden surprise caused by unexpected news or an event (literally 'water in a sleeping ear').
急に部署異動を命じられて、まさに寝耳に水だった。
I was suddenly ordered to transfer departments — it was truly a bolt from the blue.
彼の結婚発表は寝耳に水で、友人たちは驚いた。
His announcement of marriage came out of the blue and surprised his friends.
そんな噂を寝耳に水で聞かされても信じられない。
If I'm told such a rumor out of the blue, I can't believe it.