a 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. 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. This project has turned into a real thorn in our side.

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.

COMMON If someone or something is a thorn in your side or a thorn in the side, they continually annoy you or cause trouble for you. She has become a thorn in the side of the government since publishing a number of reports pointing out that public cash was being wasted. Note: You can also say that someone or something is a thorn in your flesh or a thorn in the flesh. Her mother is still a thorn in her flesh — demanding, complaining and weeping by turns. Note: This refers to a passage in the Bible, in which St Paul talks about an illness or other problem: `There was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.' (2 Corinthians 12:7) Some Pharisees, who were strictly orthodox Jews, used to deliberately hurt themselves by putting thorns in their clothes to prick them when they walked.