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.

See also: side, thorn

*thorn in someone's side

and *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.

See also: side, thorn

a thorn in your side

a thorn in the side

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.

See also: side, thorn

Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

a thorn in someone's side (or flesh)

a source of continual annoyance or trouble.

A thorn in the side comes from the biblical book of Numbers (33:55): ‘those which ye let remain of them shall be pricks in your eyes, and thorns in your sides, and shall vex you in the land wherein ye dwell’. A thorn in the flesh quotes 2 Corinthians 12:7: ‘And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure’.

See also: side, thorn

thorn in the flesh/one's side, a

A source of constant irritation. This expression appears in several books of the Bible; in the Book of Judges (2:3) it refers to enemies who “shall be as thorns in your sides,” and in the New Testament Paul refers to his infirmities as “given to me a thorn in the flesh” (2 Corinthians 12:7). The term remains current.