donner de la confiture aux cochons

To offer something valuable to someone who won't appreciate it

It's used when something good, a fine gift, good advice, quality work, is wasted on someone who won't understand or value it. It's the French cousin of 'casting pearls before swine'.

Lui expliquer la nuance, c'est donner de la confiture aux cochons, il n'écoute jamais.

Explaining the nuance to him is a waste of time, he never listens.

It's a homelier variant of 'ne jetez pas vos perles aux pourceaux' (don't cast your pearls before swine), from the Gospel of Matthew, chapter 7, verse 6.