N-COUNT (初到某地或初出茅庐却)自命不凡的家伙 You can refer to someone as an upstart when they behave as if they are important, but you think that they are too new in a place or job to be treated as important. (初到某地或初出茅庐却)自命不凡的家伙 [disapproval]
Many prefer a familiar authority figure to a young upstart.