Something that demands attention or provokes an irritated reaction.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "red flag" in a sentence
"I heard that a lot of people were getting silicone injections, which offered permanent results I know; most logical people would see a big red flag there, but not me, Little Miss Gullible."
"I went into the living room and wrote out a two-thousand-dollar check to be applied against her registration and dormitory fees for her first semester, placed a stamp on the return envelope, and walked out to the road and stuck it in the mailbox, then flipped up the red flag for the postman."
"Andrew Cunningham, a zoologist with the Zoological Society of London, recently raised a red flag about new viruses emerging from global encroachment into wildlife in an essay in the British medical journal BMJ, arguing that while this has probably happened many times in the past, such viruses failed to spread because those infected lived in remote enough areas that they either died or got well before they interacted with larger human populations."