Throughout North America, I keep seeing police cars labeled "K-9 unit". I know "K-9" is a homophone of "canine", but why don't they just use "Police dog"?
Learn English – Why does the police use “K-9 Unit” instead of “dog”
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Best Answer
The term appears to originate from the term "K-9 Corps", being a shorthand for the Army's War Dog Program established during World War II. (http://www.qmfound.com/K-9.htm).
Also, "K-9 Unit" is often used to refer to both the police dog as well as its handler. So a simple replacement with "police dog" would be insufficient.