豚ぶたに真しん珠じゅ
豚に真珠 and 猫に小判 are nearly interchangeable idioms meaning something valuable is wasted on someone who can't appreciate it, differing only in the animal imagery used.

Built from

猫ねこ+小こ判ばん

Written forms

猫ねこに小こ判ばん
Standard kanji spelling for this idiomatic expression.
ネコに小こ判ばん
Variant with ネコ in katakana; sometimes used for emphasis or stylistic reasons.
ネコにこばん

Etymology

A Japanese proverb literally meaning 'a gold coin to a cat'. The cat cannot appreciate the value of the coin, just as a person without understanding cannot appreciate something valuable. The exact origin is uncertain, but it is a well-established idiomatic expression.