His rude behavior is a major red flag.
red flag
idiomSomething that will enrage a particular person; a red rag.
She noticed a red flag when he refused to talk about his past, so she decided to be careful.
Ignoring every red flag during the initial negotiations, the investors proceeded with the merger, only to discover the company was deeply in debt.
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bleed red ink
To suffer from severe net losses.
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clap one's hands red
To clap or applaud intensely.
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cut red tape
To reduce bureaucracy.
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flag down
Use a flag or similar kind of signal to get someone's attention.
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fly the flag
To support one's country enthusiastically.
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freak flag
Unconventional or unrestrained behavior; extreme, nonconformist views; the side of one's personality which harbors a tendency toward such behavior or such views.
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Hoover flag
An empty (penniless) pocket, turned inside out, showing that someone has no money (especially during the Great Depression.)
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neither fish, flesh, nor good red herring
Synonym of neither fish nor fowl (“not easily categorized; not rightly belonging or fitting well in a given group or situation; also, not having the advantages of the various options”).