원님 덕에 나팔 분다더니 · 원님 덕에 나팔 부는 격이에요
to enjoy perks thanks to someone else's position, to ride on someone's coattails (lit. to blow the trumpet thanks to the magistrate)
Example
원님 덕에 나팔 분다더니, 언니 덕분에 나도 공연을 맨 앞자리에서 봤어요.
They say you blow the trumpet thanks to the magistrate: thanks to my sister, I got to see the show from the front row too.
Word parts
원 official, magistrate+덕 virtue; benefit+나 trumpet+팔 trumpet
Literally 'thanks to the magistrate, one blows the trumpet'. When a Joseon county magistrate (원님) traveled, trumpeters marched ahead of him and shared in the grand procession and hospitality, though the honor was really his.
Good to know
Close to 'riding on someone's coattails', but usually light and often self-mocking about an unexpected perk. For abusing borrowed power, use 호가호위 instead.