朽木不可雕 (Xiǔ mù bù kě diāo)

Meaning: Some people or materials are beyond improving.

Literally: Rotten wood can't be carved

Example: “他怎么教都学不会，真是朽木不可雕。” — However you teach him, he never learns: you can't make a silk purse of a sow's ear.

Equivalents in English, French, Spanish, Japanese, Dutch, and Polish

English: You can't make a silk purse of a sow's ear
French: Le singe est toujours singe, fût-il vêtu de pourpre (literally: A monkey is always a monkey, even dressed in purple)
Spanish: Aunque la mona se vista de seda, mona se queda (literally: Even if the monkey dresses in silk, she remains a monkey)
Japanese: 瓦は磨いても玉にならず (Kawara wa migaite mo tama ni narazu) (literally: A roof tile, however much you polish it, will not become a jewel)
Dutch: Al draagt een aap een gouden ring, het is en blijft een lelijk ding (literally: Even if an ape wears a gold ring, it's still an ugly thing)
Polish: I w Paryżu nie zrobią z owsa ryżu (literally: Even in Paris they can't make rice out of oats)

Origin: Confucius in the Analects (5.10), on his disciple Zai Yu, caught sleeping in the daytime: rotten wood cannot be carved.

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