苏秦，战国时期著名的纵横家，与张仪一同师从鬼谷子。
学成后，他首先来到秦国，向秦惠王提出连横政策。
他说：“大王的国家非常富饶，西边有巴、蜀、汉中等地丰富的物产，北方有胡地珍贵的兽皮和代地优良的马匹，南边有巫山、黔中的险峻地形作为屏障，东方则有坚固的要塞殽山和函谷关。土地肥沃，人民生活富裕，拥有万辆战车和百万勇士，还有广大的土地和丰富的物资，更有易守难攻的有利地势，真可谓天府之国。因此，秦国成为了雄霸天下的强国。再加上大王的贤能和众多的百姓，若能善用兵法谋略，完全可以兼并诸侯，消灭天下，自己称帝治理天下。我仅陈述自己的策略，希望大王能留意。”
秦惠王回答说：“我听说，鸟的羽毛不够丰满是不能高飞的；国家的法令不够完备是不能施行刑罚的；君王的道德不被推崇是不能役使百姓的；政策教化无法顺利推展是不能交给大臣其它任务的。现在先生不远千里到秦国给我建言，我会好好考虑的。”
然而，苏秦一连上书游说秦惠王十几次，但秦惠王始终没有采纳他的建议。
苏秦耗尽了身上所有的钱财，只得失意的回家。
后来，“毛羽不丰满者”这句原文被简化为“羽毛未丰”的成语，用来比喻一个人的实力还不够雄厚，无法独当一面有所作为。
Su Qin, a renowned strategist during the Warring States period, studied under Guiguzi alongside Zhang Yi.
After completing his studies, he first went to the state of Qin and proposed the policy of 'Horizontal Alliance' to King Huiwen of Qin.
He said, "Your Majesty's country is extremely wealthy. In the west are abundant resources from Ba, Shu, and Hanzhong; in the north are precious furs from Hu and fine horses from Dai; in the south are the steep terrain of Wushan and Qianzhong as barriers; and in the east are the strong fortresses of Xiaoyao and Hangu Pass. The land is fertile, the people live in abundance, with tens of thousands of chariots and millions of warriors, vast territories, rich resources, and advantageous terrain that is easy to defend and difficult to attack. It can truly be called a land of plenty. Therefore, Qin has become a powerful country dominating the world. With Your Majesty's wisdom and the many people, if you can skillfully use military strategies, you can certainly annex the feudal lords, eliminate the world, and rule the world yourself. I am only presenting my strategy, hoping that Your Majesty will pay attention."
King Huiwen of Qin replied, "I have heard that a bird cannot fly high if its feathers are not full; a country cannot enforce its laws if they are not complete; a king cannot command his people if his virtue is not respected; and policies and education cannot be implemented smoothly if they cannot be entrusted to ministers for other tasks. Now that the gentleman has come all the way from afar to give advice to Qin, I will consider it carefully."
However, Su Qin repeatedly submitted letters to persuade King Huiwen of Qin more than a dozen times, but the king never adopted his suggestions.
Having spent all his money, Su Qin returned home disappointed.
Later, the original phrase 'a bird whose feathers are not full' was simplified into the idiom 'feathers unfledged', which比喻一个人的实力还不够雄厚，无法独当一面有所作为。
