1lit. smell of breast milk not yet dried (idiom)
2fig. immature and inexperienced; still wet behind the ears
Example
suī rán虽然LucianoLucianokě néng可能yǐ jīng已经shì是chū lèi bá cuì出类拔萃，dàn但tā他hái shi还是rǔ xiù wèi gān乳臭未干。
虽然Luciano可能已经是出类拔萃，但他还是乳臭未干。
Luciano might have the crowd behind him, but he's still wet behind the ears.