It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].
Today’s kotowaza
蛙の面に水 Keru no tsura ni mizu
kaeru – a frog
tsura – a face
Mizu – water

It’s like water off a duck’s back.
Origin
A frog can live in water as well as on ground. Therefore even if someone put much water on a frog, it does not affect him.
Its meaning
Even if someone says such a awful thing or does awful this, it does not matter at all. Sometimes this Japanese saying is used to describe someone who is insensitive or has nerves of steel.
An example
Even if I say to my sister 「Can you do something about your funny hair style?」 Kaeru no tsura ni mizu.

arigato