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