Derogatory or teasing term for an inexperienced young person, likening them to a child who still smells of milk. Somewhat literary or old-fashioned; not common in everyday conversation.
He's still a greenhorn, so we can't entrust him with important work.
What could such a fledgling possibly understand?
Compound of 乳 (milk) + 臭 (smell) + 児 (child), literally 'milk-smelling child'. The exact historical derivation is uncertain, but the term has been used figuratively since at least the Edo period.