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.
例句
Many prefer a familiar authority figure to a young upstart.
很多人都更愿意跟自己熟悉的权威人物打交道，而不是一个年纪轻轻却自命不凡的家伙。
...an upstart who had come from nowhere.
原本是个无名之辈的目中无人的新丁