A warning sign or indicator of potential danger, problems, or concerns that require attention.
From maritime and railway signaling systems (1700s-1800s) where red flags indicated danger or stop. The metaphorical usage emerged in the early 1900s, maintaining the universal association of red with warning or danger.
This phrase taps into one of humanity's most primal color associations - red as danger - making it instantly understood across cultures. In business, calling something a 'red flag' is more urgent than saying 'concern' because it triggers our evolutionary fight-or-flight response.
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