強弩之末

From a memorial submitted by Han Anguo in 135 BCE included in Shiji and Book of Han:

漢數千里爭利，則人馬罷，虜以全制其敝。且彊弩之極，矢不能穿魯縞；衝風之末，力不能漂鴻毛。非初不勁，末力衰也。擊之不便，不如和親。 [Classical Chinese, trad.]

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汉数千里争利，则人马罢，虏以全制其敝。且强弩之极，矢不能穿鲁缟；冲风之末，力不能漂鸿毛。非初不劲，末力衰也。击之不便，不如和亲。 [Classical Chinese, simp.]

From: 135 BCE, 韓安國, 《匈奴和親議》

Hàn shùqiānlǐ zhēng lì, zé rénmǎ pí, lǔ yǐ quán zhì qí bì. Qiě qiáng nǔ zhī jí, shǐ bùnéng chuān Lǔ gǎo; chōngfēng zhī mò, lì bùnéng piào hóngmáo. Fēi chū bù jìng, mò lì shuāi yě. Jī zhī bùbiàn, bùrú héqīn. [Pinyin]

If the Han [troops] go several thousand li to contend for advantage, our men and horses are to be exhausted, which will be taken advantage of by the caitiffs with the fullness of their strength. Moreover, at the extremity of the range of a strong crossbow, the arrow cannot pierce the [light] silk of Lu; at the end of a gust of wind, its strength cannot even cause a goose feather to float. Not that they are not powerful in the beginning, but their strength has declined at the end. It is not expedient to attack them, [thus] it would be better to build friendly relations through royal marriage.

彊 is the archaic variant of 強／强.

強弩之末

1. (figurative) exhausted strength; spent force; strong force that has been exhausted