also: カエルのつらにしょうべん
expression
like water off a duck's back
Idiomatic expression meaning that criticism or scolding has no effect on someone, just as urine on a frog's face would not bother it. Colloquial and somewhat vulgar due to the word 小便.
See also: 蛙の面に水
あいつに何を言っても蛙の面に小便だよ。
No matter what you say to that guy, it's like water off a duck's back.
注意しても蛙の面に小便で、全然聞いていない。
Even if you warn him, it's like water off a duck's back; he doesn't listen at all.
The phrase literally means 'urine on a frog's face.' Frogs are often associated with water and are thought to be unfazed by getting wet, so the image of urine—a liquid—on its face suggests that criticism or scolding has no effect. The exact origin is uncertain, but it is a colloquial idiom.