thorn in side

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
Farlex Dictionary of Idioms.

*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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

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.
See also: flesh, thorn
The Dictionary of Clichés by Christine Ammer
See also:
  • a thorn in your side
  • a thorn in (one's) side
  • a thorn in someone's side
  • be a thorn in (one's) flesh
  • be a thorn in (one's) side
  • be a thorn in your flesh/side
  • become a thorn in (one's) flesh
  • become a thorn in (one's) side
  • a thorn in the flesh
  • it/that sucks