man-of-war
man-of-war {n. phr.}
1. A stinging jellyfish in the Gulf of Mexico.
"No swimming today," the sign said, "as we have a man-of-war alert."
2. A large battleship displaying its national flag or insignia.
The shipwreck victims were picked up by an American man-of-war.
3. The albatross bird.
After several days at sea we noticed some men-of-war majestically sailing through the skies.
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Source: A Dictionary of American Idioms