What Does "A Fly In The Ointment" Mean?

A fly in the ointment is an old idiom still in use today. It means one problem or unpleasant element in a situation which is otherwise perfect, or at least very good. For example, you might say: "I love my job, and my colleagues are really nice. The only fly in the ointment is the boss - he's so difficult." This would imply that your boss is the only thing you don't like about your job.

The idea behind this idiom is that ointment is something good - you use it to soothe painful skin or improve your appearance, so basically a smooth, peaceful situation can be compared to ointment. On the other hand, flies are associated with dirt, disease and general unpleasantness. If you have a fly in your jar of ointment, the jar is contaminated and you will not want to use it again, or at least until you've got rid of the fly. So it really is quite a logical idiom.

The expression is derived from Ecclesiastes 10:1. The essence is that a small thing that is bad, (dead and rotting flies) can overwhelm what is otherwise pleasant or good (a perfumer's ointment). The second half of the verse states that "a little folly outweighs much wisdom."

Many a hard-won reputation has been destroyed by being caught in a single indiscretion.

In English, the phrase fly in the ointment is an idiomatic expression for a drawback, especially one that was not at first apparent, e.g.
Sam's lack of map-reading skills turned out to be the fly in the ointment when he applied for the job.
A likely source is a phrase in the King James Bible:[1]
Dead flies cause the ointment of the apothecary to send forth a stinking savour (Ecclesiastes 10:1)
For five centuries now 'a fly in the ointment' has meant a small defect that spoils something valuable or is a source of annoyance. The modern version thus suggests that something unpleasant may come or has come to light in a proposition or condition that is almost too pleasing; that there is something wrong hidden, unexpected somewhere. This idiom has been used in the title of some books: The Fly in the Ointment: 70 Fascinating Commentaries on the Science of Everyday Life and The Fly in the Ointment by Alice Thomas Ellis.