thorn in side
thorn in (one's) side
A constant or persistent cause of annoyance, frustration, or trouble. My professor was an environmental activist when he was younger, and, apparently, he was quite a thorn in the oil companies' sides at the time. My little brother is such a thorn in my side—he always wants to do stuff with me, especially when I'm trying to hang out with my friends.
*thorn in someone's sideand *thorn in someone's flesh
Fig. a constant bother or annoyance to someone. (*Typically: be ~; become ~.) This problem is a thorn in my side. I wish I had a solution for it. John was a thorn in my flesh for years before I finally got rid of him.