寝耳

[mella]

そりゃあ寝耳にミミズ寝耳に水だなア
I think it is a sort of idiomatic expression. I cannot understand the meaning.
"I aked myself if I would have heard the water while sleeping without hearing while sleeping"?

[NileCat]

It’s just a pun. Mizu(water) and Mimizu(earthworm).
And the idiom means “a bolt from the blue”.

　That’s a bolt from the ___”, I mean, a bolt from the blue.

I couldn’t come up with a good pun.
Anyone?