be a thorn in the flesh (for one)

To be a constant or persistent cause of annoyance, frustration, or trouble (for one). My little brother is such a thorn in the flesh—he always wants to do stuff with me, especially when I'm trying to hang out with my friends. My professor was an environmental activist when he was younger, and apparently he's been a thorn in the flesh for oil companies ever since. This project has turned into a real thorn in the flesh.