Red flag
Etymology
An old idiom from the literal red flag used as a danger or stop signal (and historically as a banner of revolt). It was revived and sharpened in 2020s dating discourse, often paired with the 🚩 emoji, and extended by analogy into green flags and beige flags.
noun · /RED flag/
A warning sign of likely trouble in a person, relationship, or situation.
What it names
It names the early indicator you ignore at your peril: the behavior that, in hindsight, predicted the problem. In dating discourse it has become a shorthand for triaging people quickly — cataloguing the traits that should give you pause before you’re invested.
Example
Speaking to the waiter that rudely on a first date is a red flag I can’t unsee.
Related idea
The opposite of a green flag: one warns you off, the other reassures.