羽毛未丰 undeveloped feathers
Explanation
比喻年纪轻，缺乏经验，不成熟。
It means to be young, lack experience and immature.
Origin Story
战国时期，苏秦游说诸国，前往秦国游说秦惠王，希望秦国与其他六国联盟。秦惠王野心勃勃，想吞并六国，认为苏秦年少轻狂，不予理会，说苏秦是‘羽毛未丰’的小鸟，难以成就大业。苏秦并没有气馁，继续游说其他国家，最终成功促成了合纵抗秦，成为赫赫有名的纵横家。
During the Warring States period, Su Qin lobbied various states and went to the state of Qin to persuade King Huiwen to form an alliance with the other six states. King Huiwen, however, was ambitious and wanted to conquer the six states. He ignored Su Qin, saying that he was a 'young bird with undeveloped feathers', incapable of great achievements. Su Qin was not discouraged; he continued to persuade other states. Eventually, he successfully formed an alliance against Qin and became a famous strategist.
Usage
形容人年轻，缺乏经验，不成熟。常用于比喻句中。
It is used to describe someone who is young, inexperienced, and immature. Often used in metaphors.
Examples
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他经验不足，正值羽毛未丰之时，不宜承担如此重任。
ta jingyan buzu,zheng zhi yumai weifeng zhishi,buyitai chengdan ruci zhongren
He is inexperienced and still young, so he shouldn't take on such a heavy responsibility.
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小明羽毛未丰，做事总是毛手毛脚的。
xiaoming yumai weifeng,zuoshi zongshi mao shou mao jiao de
Xiaoming is inexperienced and always clumsy in his work.