In Chinese, the word "菜鸟(cài niǎo)" (newbie or green hand in English) refers to a person who is new in a team or organization or inexperienced at something. The person who just starts a new career and needs some time to adapt to the environment and new things is called "菜鸟" in Chinese.
中文里的"菜鸟"（英文为newbie or green hand）一词是指某个人刚刚加入某个团队，或某个组织，对于某事还欠缺经验。刚刚进入某个事业，适应环境、接受新的事物，都是需要一段时间的，就叫菜鸟。
From the literal meaning of "菜鸟 (cài niǎo)" (vegetable bird), it seems to refer to the bird that feeds on vegetables alone. But the fact is, the meaning derives from something completely different. Let's take a look at the origin of "菜鸟 (cài niǎo)".
People used to raise pigeons for entertainment rather than food in the beginning. Later, some people started to raise pigeons for food and they called these pigeons "肉鸽 (ròu gē)" (meat pigeon) or "菜鸽 (cài gē)" (vegetable pigeon) in order to distinguish them from the pigeons before. Others like "肉鹌鹑 (ròu ān chún)" (meat quail) and "菜鹌鹑 (cài ān chún)" (vegetable quail) with "菜鸽 (cài gē)" raised for food are generally called "肉鸟 (ròu niǎo)" (meat bird) or "菜鸟 (cài niǎo)" (vegetable bird). That is where "菜鸟 (cài niǎo)" comes from.
There is one line of thought claiming "菜鸟 (cài niǎo)" was originally written as "笨鸟 (bèn niǎo)" (stupid bird), because someone mistook "菜" as "笨". He was mocked by others and this resulted in "菜鸟 (cài niǎo)", which eventually came into popular use. Another explanation is that the word "菜鸟 (cài niǎo)" originated from NBA basketball circles, referring to a team newbie who needs some time to understand the rules and get experience in play.