黔驢技窮 qiánlǘjìqióng: someone who has exposed his limited ability; the (proverbial) Guizhou donkey has exhausted its tricks—at one's wit's end; at the end of one's rope. This idiom comes from the following political fable by Liu Zongyuan (AD 773–819):
Source here. Translation:
There were no donkeys in Qian until someone who was fond of curiosities brought one in by boat. After the man got it there, he found the donkey was useless, so he let it loose near the hills. A tiger, upon seeing it, thought it was such a large beast that it took it for a god. So the tiger hid in the forest to spy on it. Bit by bit the tiger came closer to it, but carefully so that it wouldn't know.
One day the donkey brayed, and the tiger was so terrified that he ran far off. He thought that the donkey was going to eat him and was extremely frightened. Yet as the tiger kept observing it time and again, he realized there wasn't anything unusual about the donkey. The tiger had gotten increasingly used to hearing the braying. He now came out near the donkey circling it, but still dared not pounce. In a little while, he pressed even closer to it, and he nudged it unconcernedly. Overcome with rage, the donkey kicked out at the tiger.
Now the tiger happily reckoned to himself, "So this is the extent of its talents." Thereupon he leaped, roaring loudly, and ripped open the donkey's throat. He ate his fill and then left.
Alas! The donkey's larger size made it seem to be a creature of virtue; its loud voice made it seem to be a creature of ability. If it had never revealed the limit of its talents, the tiger, despite his own ferociousness, would still have been suspicious and fearful and in the end would not have dared attack it. Now, instead, things have come to this—how disheartening!
Translated by Madeline K. Spring