It means that a person who is innocent is innocent even if he doesn't clarify himself; And the person who is a bad person himself, even if he denies a bad thing he has done in every way, he is still a bad person at heart.
The meaning of this sentence is: In the face of the test of external environment, people with good potential will naturally behave as good people, while those with bad nature will naturally develop to the bad side. Explain that people or things naturally show their true colors in certain environmental changes.
The allusion comes from Qu Yuan's The Fisherman.
The full text is as follows:
Qu Yuan, once released, swam in Jiangtan and sang songs on the banks of Ze River, with haggard color and withered description. When the fisherman saw this, he asked, "Is the son not from Dr. San Lv? Why is it? " Qu Yuan said: "The whole world is turbid and I am alone. Everyone is drunk and I wake up alone, so I can see it." The fisherman said, "A saint does not cling to things, but can go with the times. The world is muddy, why not spit out its mud and raise its waves? Everyone is drunk, why not feed the bad and suck it? Why do you think deeply and hold high, and let yourself be? " Qu Yuan said, "I heard that the new bather will play the crown and the new bather will shake his clothes;" Ann can observe with her own body, but what about the people who receive things? I would rather go to Xiangliu and be buried in the belly of a river fish. Ann can be white and covered with worldly dust? " The fisherman smiled and went away with a drum, but he sang, "The water in the surging waves is clear, and you can hold my tassel;" The water in the surging waves is turbid, and you can lick my feet. " Then go, no longer with words.