毛máo遂suì自zì荐jiàn

Literal: Mao Sui recommends himself
Figurative: To volunteer oneself for a task or position on one's own initiative, without being asked; to step forward confidently to take on a responsibility.

Where does 毛遂自荐 come from?

When the state of Qin laid siege to Handan, Zhao's capital, Lord Pingyuan needed twenty capable men for an urgent embassy to Chu, but he could find only nineteen who met the mark. A little-known retainer named Mao Sui then stepped forward and offered himself to complete the group. Pingyuan hesitated, since in the three years Mao Sui had served him he had heard nothing of the man's merits; Mao Sui retorted that this was only because he had never been placed in the bag, and Pingyuan agreed to take him. At the Chu court, when the talks stalled, it was Mao Sui who, sword in hand and with bold words, persuaded the king of Chu to ally with Zhao. On their return, Pingyuan admitted he had misjudged the man's worth.

Source (出处): 《史记·平原君虞卿列传》

How is 毛遂自荐 used? Two examples

公gōng司sī要yào派pài人rén去qù谈tán这zhè个ge项xiàng目mù，他tā毛máo遂suì自zì荐jiàn，主zhǔ动dòng接jiē下xià了le任rèn务wù。
The company needed to send someone to negotiate this project, and he volunteered himself and took on the task.
没méi有yǒu人rén愿yuàn意yì当dāng组zǔ长zhǎng，于yú是shì小xiǎo李lǐ毛máo遂suì自zì荐jiàn站zhàn了le出chū来lái。
Nobody wanted to be the group leader, so Xiao Li volunteered himself and stepped forward.

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