noun
greenhorn; inexperienced youth
Literary or somewhat old-fashioned term for a young, inexperienced person, likened to a chick with a yellow beak. Often used in a mildly derogatory or pitying way.
彼はまだ黄口だから、大事な仕事は任せられない。
He's still a greenhorn, so I can't entrust him with important work.
黄口の若造が生意気な口をきくな。
Don't talk back, you inexperienced young brat.
From the image of a chick's yellow beak, symbolizing youth and inexperience. The exact historical derivation is uncertain, but the spelling is conventionally associated with this meaning.