What is the origin of the idiom "a thorn in one's side" and when to use it?

The phrase "thorn in one's side" originates from the Bible in the New Testament, specifically the letters of Paul. In 2 Corinthians 12:7, Paul mentions having a "thorn in the flesh" that was given to him as a means of keeping him humble. The exact nature of Paul's thorn is not specified in the biblical text, leaving room for interpretation. However, it is commonly understood as a metaphorical representation of a chronic difficulty or adversity he faced. Over time, the phrase has evolved and is now used more generally to describe any ongoing source of irritation, annoyance, or trouble that is difficult to resolve or remove.