a thorn in someone's side

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