Make a mountain out of a molehill is a very old English idiom. It has been used since the 1500s and means to treat a small problem as if it were a huge one.
The image is easy to understand once you know what a molehill is. A mole is a small animal that digs underground, leaving a little mound of earth on the surface. A mountain is enormous by comparison. So the expression creates a strong contrast between something tiny and something massive.
When someone says a person is making a mountain out of a molehill, they mean the reaction is bigger than the actual issue.
- Missing one email becomes a crisis.
- A small mistake feels like total failure.
- A minor delay is treated like a disaster.
Writers and speakers have used similar comparisons for centuries because the picture is so clear. Even today, the phrase remains popular because it quickly shows the idea of exaggeration in a memorable way.