Learn the meaning and usage of the term " red flag " as a noun and a verb, with synonyms, antonyms, and related terms. Find out the origin, pronunciation, and examples of " red flag " in different contexts. A red flag means a warning or indication of potential dangers or problems. Imagine you’re about to buy something online, but several reviews mention the product falling apart within a week. That’s your red flag waving furiously! This idiom comes from a more literal origin: actual red -colored flags that are used to warn others. An idiom is a phrase or expression whose meaning cannot be understood from the ordinary meanings of its individual words. They’re important because they make ... RED FLAG meaning : 1. a flag used as a sign of danger or to tell someone in a race to stop: 2. a flag used as a…. Learn more. The meaning of RED- FLAG is to identify or draw attention to (a problem or issue to be dealt with). How to use red- flag in a sentence.