Sit by a stump waiting for more hares to come and dash themselves against it -- Trust to chance and windfalls

Long time ago, in the State of Song, there was a farmer in whose fields stood a tree stump. One day when he was ploughing in the fields, a hare came dashing past. Running too fast, it dashed against the stump and broke its neck, died instantly. The farmer stopped hoeing and picked up the dead hare. He was overjoyed at the unexpected gain. He took the hare back home, cooked it and had a nice dinner.

After that he thought, It comes so easily! I'm tired of farming. Why not just sit under the tree and wait for more hares to run into the trunk!

So the farmer abandoned his plough and waited by the tree stump every day, indulging himself in the fantasy that he would get another hare. He waited and waited. Many days passed but nothing happened. He did not get his hare but became a laughing stock in the state, and his field was soon overgrown with weeds.

The fable was summed up in the idiom. This metaphor is used to mock people who attempt accidental gains without pains -- those who are not willing to make any effort but trust mere luck.