かえる つら みず
kaeruno tsurani mizu

Idiomatic Expression

1. like water off a duck's back; water on a frog's face — literal

まさに かわず めん みずとはこのことだろう。
masani kaeruno menni mizutowa kono kotodarō.
That's a clear case of "water off a duck's back".
Other forms: