• RED FLAG (noun). The noun RED FLAG has 3 senses: 1. a flag that serves as a warning signal. 2. the emblem of socialist revolution. 3. something that irritates or demands immediate action. Familiarity information: RED FLAG used as a noun is uncommon.. a flag that serves as a warning signal. 2. the emblem of socialist revolution. 3. something that irritates or demands immediate action. Familiarity information: RED FLAG used as a noun is uncommon.