ねみみにみずne mimi ni mizu
Like water in your sleeping ear? That's one theory (according to this website), but the more likely explanation is that the 耳 in this case is used in the same sense as 耳にする: to hear.
Definition:
不意の出来事に、ひどく驚くことのたとえ。
Translations:
1. Like a bolt out of the blue.
2. Like a flash-flood
As the site explains (see reference above), if you imagine the water to be a flooding river, you get the image of suddenly being able to hear (or being awakened) by the sound of rushing water coming your way. This, more than the idea of having water poured into your ear, captures the sense of fearful shock that this expression is used to convey.
例文：私の主人がパチンコで私たちの貯金を全部使い果たしたことを初めて聞いた時は寝耳に水だった。
When I found out that my husband lost all of our savings at pachinko, it was like being caught unawares in the raging waters of a flash-flood.
