This idiom describes a small flaw, problem, or annoyance that spoils an otherwise perfect or promising situation. The ‘fly’ represents a minor but disruptive element, and the ‘ointment’ symbolizes something valuable, like a plan or experience. The phrase is used to highlight how small issues can undermine larger successes.

The phrase comes from the Bible, specifically Ecclesiastes 10:1 (King James Version, 1611): ‘Dead flies cause the ointment of the apothecary to send forth a stinking savour.’ The verse warns that small errors can ruin great works. By the 17th century, it was adapted into English vernacular, with John Ray’s 1678 *English Proverbs* recording it. Its use grew in the 19th century, particularly in British and American literature, to describe minor but aggravating flaws, as seen in Charles Dickens’ works.