Another frog kotowaza! Because… I don’t know why. Don’t let it bother you.
(Kaeru no tsura ni mizu; “Water in a frog’s face”)
Not being bothered by things at all. No matter what happens, keeping one’s composure, or even not showing a reaction at all. The vicissitudes of life, to you, are like water to a frog – and frogs live in the water. This can be functionally equivalent to the English expression “Like water off a duck’s back.”
Note, however, that the saying is often used in a critical way, to describe someone’s impudent or shameless behavior. In that case, the “water” that doesn’t draw a response is the disapproval or censure of the people around the “frog,” who is acting as they please without consideration for others.
蛙 is “frog” again, although for this saying the pronunciation is the usual kaeru. の is, as always, the associative particle, here serving its familiar possessive function. 面 (tsura) is “surface” or “face” (apparently it used to refer specifically to cheeks, as well). に (ni) is a directional particle modifying 面 (giving us “in the face” or “into the face”), and finally 水 (mizu, “water”). Reverse the word order and you get “water in face of frog.”
Apparently the Japanese observed that if you splash water on a frog, even in its face, it doesn’t react. I haven’t tested this empirically (and it might be hard to, given that they’re likely to be freaking out about your presence). My sources note that this expression doesn’t apply to people who are too magnificent to be bothered by something trivial; instead, it’s for people who remain calm when most in their position wouldn’t.
The “water” (水) can also be replaced with 小便 (shouben, “urine”). Make of this what you will.
(“Donna ni hihan sarete mo, kare wa ki ni sezu ni shitai koto wo suru. Kaeru no tsura ni mizu no you da.”)
[“No matter how much he’s criticized, he just does what he wants without a care in the world. It’s like throwing water in a frog’s face.”]