This idiom means being self-contradictory.
Chǔguó yǒu gè mài máo hé dùn de rén
楚国Chǔguó🔊有个yǒu gè🔊卖mài🔊矛máo🔊和hé🔊盾dùn🔊的de🔊人rén🔊。
Tā chuīxū shuō wǒ de dùn fēicháng jiāngù shénme máo dōu cì bù chuān
他tā🔊吹嘘chuīxū🔊说shuō🔊：我的wǒ de🔊盾dùn🔊非常坚固fēicháng jiāngù🔊，什么shénme🔊矛máo🔊都dōu🔊刺不穿cì bù chuān🔊。
Yòu shuō wǒ de máo fēicháng fēnglì shénme dùn dōu néng cìchuān
又说Yòu shuō🔊：我的wǒ de🔊矛máo🔊非常锋利fēicháng fēnglì🔊，什么shénme🔊盾dùn🔊都dōu🔊能néng🔊刺穿cìchuān🔊。
Yǒurén wèn yòng nǐ de máo cì nǐ de dùn huì zěnme yàng ne
有人Yǒurén🔊问wèn🔊：用yòng🔊你的nǐ de🔊矛máo🔊刺cì🔊你的nǐ de🔊盾dùn🔊会huì🔊怎么样zěnme yàng🔊呢ne🔊？
Tā yǎkǒu wúyán dá bù shànglái
他tā🔊哑口无言yǎkǒu wúyán🔊，答不上来dá bù shànglái🔊。
English Translation:
In Chu, there was a man who sold spears and shields. He boasted: "My shield is so sturdy that no spear can pierce it." He also said: "My spear is so sharp that it can pierce any shield." Someone asked: "What happens if you use your spear to pierce your shield?" He was speechless and could not answer.