自zì相xiāng矛máo盾dùn

Literal: one's own spear against one's own shield
Figurative: To contradict oneself; to hold two mutually incompatible claims at the same time.

Where does 自相矛盾 come from?

In the state of Chu, a man selling weapons hawked his wares. Of his shield he said, "It is so strong that nothing can pierce it." Of his spear he said, "It is so sharp that it pierces anything." Then someone asked him, "What happens if you use your spear against your shield?" The man had no answer, for both claims could not be true at once.

Source (出处): 《韩非子》

How is 自相矛盾 used? Two examples

他tā刚gāng说shuō自zì己jǐ没méi空kòng，转zhuǎn头tóu又yòu说shuō想xiǎng去qù逛guàng街jiē，前qián后hòu自zì相xiāng矛máo盾dùn。
He just said he had no time, then a moment later said he wanted to go shopping; from start to finish he contradicts himself.
这zhè份fèn报bào告gào的de数shù据jù自zì相xiāng矛máo盾dùn，前qián面miàn说shuō增zēng长zhǎng，后hòu面miàn又yòu说shuō下xià降jiàng。
The figures in this report contradict one another: earlier it says there was growth, later that there was a decline.

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