inexperienced person; novice; greenhorn
Often used self-deprecatingly or to describe someone lacking skill or experience. Can carry a humble or slightly critical tone.
I'm still a novice, but I'll do my best.
He's a greenhorn, so please teach him a bit more gently.
Compound of 未熟 (immature, unskilled) and 者 (person). The exact historical derivation is uncertain, but the meaning is transparent from its components.