a thorn in someone's 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.
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’.