become a thorn in the side

To become a constant or persistent cause of annoyance, frustration, or trouble. My little brother is becoming such a thorn in the side. He always wants to do stuff with me, especially when I'm trying to hang out with my friends. Did you know that Jenny is an environmental activist? Yeah, I hear she's become a real thorn in the side for oil companies. This lawsuit has really become a thorn in the side. Let's just settle out of court.