expression, suru verb (irregular)
act self-important; be arrogant; throw one's weight around
Colloquial expression for behaving arrogantly or as if one is more important than others. Often used in negative contexts to criticize someone's attitude.
Ever since he got promoted, he's been acting all self-important.
あいつはいつもでかい顔をして、人の話を聞かない。
That guy always acts high and mighty and never listens to anyone.