wet behind the ears源于胎生动物，如牛、羊刚生下来的时候，全身是湿漉漉的；虽然它们的身体很快就干了，但是耳朵背后却还是湿的。如果有人对你说，你耳朵背后是潮的，可能就是在嘲笑你像新生儿一般幼稚无知，即“年轻且无经验的，缺乏经验，乳臭未干，少不更事”。

【例句】
Look, I want an experienced teacher, I don't want someone who is wet behind the ears.
听着，我想要一个经验丰富的老师，不想要一个乳臭未干的人。

The committee took no notice of what Peter said; obviously1 they thought he was still wet behind the ears.
委员会对彼得说的话不屑一顾，显然他们认为他缺少经验。