something that persistently bothers, irritates, or causes pain
—I like all of the committee members except Jason. He disagrees with everything I say and has really become a thorn in my side.

As the Israelites entered Canaan, God told them to force out all the people living there:

But if you do not drive out the inhabitants of the land before you, then those whom you allow to remain will be irritants in your eyes and thorns in your side [thorns in your sides—KJV], and will cause you trouble in the land where you will be living (Numbers 33:55).

Much later, Paul uses a similar phrase to describe a personal problem. After writing about a vision that he received from God, he continues,

In the New Testament, Paul writes in his second letter to the Corinthian church that he has seen a vision from God but adds that it would be wrong for him to boast or become proud:

Therefore, so that I would not become arrogant, a thorn in the flesh was given to me, a messenger of Satan to trouble me—so that I would not become arrogant. I asked the Lord three times about this, that it would depart from me. But he said to me, “My grace is enough for you, for my power is made perfect in weakness.” (2 Corinthians 12:7-9)

Though there have been many ideas about what Paul’s “thorn” is, Paul does not say.