boyishness; inexperience; greenness
Figurative expression likening youthful inexperience to the lingering smell of milk. Often used in set phrases like 乳臭い (にゅうしゅうくさい) or 乳臭児 (にゅうしゅうじ).
His opinions still have a trace of inexperience.
It's a job that can't be left to a greenhorn.
greenhorn; fledgling; novice
Compound of 乳 (にゅう, 'milk') and 臭 (しゅう, 'smell'), literally 'milk smell'. Figuratively extended to mean boyishness or inexperience, from the idea that a child still smells of milk.