乳臭未干

rǔ xiù wèi gān

lit. smell of breast milk not yet dried (idiom)

fig. immature and inexperienced; still wet behind the ears

乳

rǔ

(bound form) breast

臭

xiù

sense of smell

未

wèi

not yet

干

gān

(bound form) to have to do with; to concern oneself with