Johnny-come-lately
noun /ˌdʒɒni kʌm ˈleɪtli/
/ˌdʒɑːni kʌm ˈleɪtli/
[singular] (disapproving or humorous)- a person who has only recently arrived in a place or started an activity, especially somebody who is more confident than they should be
(尤指自负的)新人,新手