The term "red flag" is a common idiomatic expression that means something is wrong or there are warning signs indicating potential danger, trouble, or risk. It can also refer to an obvious sign of a problem or a clear indication that something is not right. The phrase originated from the use of red flags as signals for different purposes such as starting a race, indicating a hazard on a golf course, or warning of danger in various situations. In essence, when someone says "red flag," it means there are signs of potential trouble ahead and caution should be exercised.