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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"?
A police dog, also known as K-9 or K9 (a homophone of canine), is a dog specifically trained to assist members of law enforcement. - nationalpolicedogfoundation.org
Origin of the term 'K-9 unit'
An Elephind newspaper database search turns up at least one instance of "K-9 unit" from World War II. From "5th Infantry Dog Up to Snuff With '201' File" in the [Colorado Springs, Colorado] Camp Carson Mountaineer (November 11, 1943):
It is not unusual for a soldier to have a personal (201) file in Regimental Headquarters. This is standard operating procedure. Not is it news when a dog bites a man. Bu this is a case where the dog involved did not bite anyone—but has a file (201) all his own.
The canine in question answers to the name of Charley. He is owned and operated by the Regimental Adjutant, Capt. Adelbert Boggs. Charley i a fine, year-old Doberman Pinscher. He joined the Regiment way down yonder at Camp Van Dorn.
The file keeper, S/Sgt. Saul Kash is in charge of Charley's 201 file as well as those of the entire Regiment. He says that Charley's dossier is quite complete. It contains Charley's admittance date to the Fifth—and very valuable documents surrounding the pup's efforts to join the fighting K-9 unit. This, as you know is the corps of trained dogs which is proving of such invaluable assistance to our sentries everywhere.
Newspaper references to "K-9 corps" during the same year are far more numerous, however. Here are the earliest three such instances. From "Two Coronado Dogs Accepted by Army's 'Dogs for Defense'," in the Coronado [California] Journal (January 7. 1943):
Forty-one San Diego county dogs to date have been accepted for service in the new K-9 corps of the United States Army, it was announced by officials of Dogs for Denfense, Inc., in charge of recruiting of the canine-soldiers in San Diego and Imperial counties.
"Denfense" appears to be a typo, but with stories like this one, you never know.
From "Falla's a Private," in the [Newburgh, New York] Stewart Field United States Military Academy Prop Wash (March 9, 1943):
Falla, President Roosevelt’s Scottish terrier, has gone to war. He hasn’t joined the K-9 Corps, for he isn’t big enough. But he’s a private, thanks to the President’s personal $1 donation to the War Dog Fund. The dollar entitled Falla to the rank of either an Army private or a Navy seaman, and the President asked that Falla be made a private. War Dog Fund has turned over $15,000 so far to Dogs for Defense, national procurement agency of dogs being used by the various military services at home and abroad.
From an untitled item in the Suttons Bay [Michigan] Courier (April 8, 1943):
They’re in the army now—bow wow, and how! Tens of thousands of America’s dags have gone to war, in the K-9 corps. Read of the splendid job they’re doing and how they’re trained, as told by Louis de Casanova, former editor of the Kennel Club Gazette, in The American Weekly, the magazine distributed with next week's Chicago Herald-American.
None of these early instances explains the origin of the name—but presumably that's because it was obviously chosen as a visual pun—making the verbal connection between the word canine and the military inclination to organize individuals (whether people or dogs) into groups with numerical IDs. The fact that "K-9 corps" and "K-9 unit" were appearing in newspapers across the country—from New York to Michigan to Colorado to California (along with instances in newspapers from Indiana, Texas, Virginia, and Hawaii)—within the year 1943 indicates that the name caught the fancy of working journalists (and evidently the U.S. public as well). I didn't find any mentions of "K-9 unit" or "K-9 corps" in the Elephind database from 1942 or earlier.
Man-dog team or police department subunit?
It seems worth noting that "K-9 unit"—like "K-9 corps"—originally applied to a section or group of trained military dogs, not to a particular pairing of a dog and its handler. In modern policing, however, people sometimes apply the the term "K-9 unit" to a man-dog team and sometimes to the unit of the police force that consists of such teams. Thus, for example, from "Local Man Fires Shot at Burglars, Suspects Escape Police Dragnet," in the Louisville, Colorado Times (December 5, 1990, page 11) we have an instance of the "man-dag team" sense of the term:
Louisville police also requested a K-9 unit from the Boulder County Sheriffs Office, but no dog units were available for the search. Somehow the two young burglars managed to elude the dragnet. Glass said their escape was especially frustrating, as they may be linked to burglaries in Broomfield and Longmont.
But far more common in Elephind's newspaper matches is the term in its departmental sense, as in this instance from Shawn Dockry, "Kent Police Praise Dog Unit," in the [Kent, Ohio] Daily Kent Stater (April 5, 1991):
The Kent City Police and the Portage County Sheriffs departments literally want to take a bite out of crime with their respective K-9 units.
Portage County Sheriff P. Ken Howe said his dog unit started six months ago with one dog, Erd, pronounced 'Ed.' A new dog was added to the unit in March.
"The dogs are to us what the horses are to Cleveland (police units)," Howe said.
The creation of the K-9 unit was a campaign promise of 1988, Howe said. The dogs are used to track lost people and suspects, to sniff out drugs and to back up officers.
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.
Oddly enough, there is an early sci-fi movie called Just Imagine that was released in 1930. It depicts the future of New York City in 1980 where humans are referred to by numbers. The dog is referred to as K9 even with a tag on it that reads K9. I wonder if this is where it all started.
The origin of the term K-9 goes back as early as 1857; and it was first used as a number assigned for a stray dog, per Green's Slang Dictionary.
1. a dog.
Sergeant Watkinson, K 9 (certainly a most appropriate number) deposed to having discovered the stray pet [...] and then and there, spite of the dog’s animosity to all police [...] captured the brute.
Bell’s Life in Sydney 2 May 3/4
Update: In the above citation, I've originally thought the number K 9 was also assigned to the stray dog by the sergeant because K 9 sounds like canine and GDoS provides the citation under the meaning "a dog". Sven Yargs has found additional valuable information from Australian archive below where it is mentioned that K 9 is the badge number of the sergeant; and the original usage of K 9 is connected to the police department and the dog indirectly:
It appears from "Police Pickings" in Bell's Life in Sydney, and Sporting Reviewer (May 2, 1857) that "K 9" is the sergeant's badge number, although the reporter makes the humorous connection: "K 9 [Watkinson], of course, knew nothing about such vermin, not he—he wasn't a dog fancier—not he; his letter and number did not quality him as such, though it did read canine." Sergeant Watkinson appears in multiple Sydney newspaper articles from 1855 to 1857, but never again as "K 9."
After that, the term was adopted by U.S Army first for the War Dog Program (K-9 Corps) during World War II and then it was adopted for the police dogs.
From 2000, the term has the sense 'a corrections officer' also, per Green's Slang Dictionary:
K-9 Corrections officer (canine).
Other Side of the Wall: Prisoner’s Dict. July
Further read for the history of K9 breeds:
K9 Working Breeds: Characteristics and Capabilities By Resi Gerritsen, Ruud Haak
1) "K-9" or "K9" -- a typographic pun upon "canine", inherited from military jargon. They could have said canine, or dog (though someone would surely then complain that they weren't giving bitches equal credit). Someone just happened to like this bit of jargon, and it stuck.
2) "K9 Unit" -- The "unit" isn't just the animal, but the team of the animal and its handler.
(The term sometimes also includes the vehicle, when it has been specially modified for this purpose. For example, the local K-9 units have the handler carry a transmitter which can remotely release the dog from their van. They do "live training" demos of this at some of the local fairs, showing how quickly the dog can be out of the van and assisting. I'm not sure whether the transmitter has a deadman/man-down sensor or if the officer has to explicitly press a button; if I was designing it, it would support both modes.)
Additional: in the UK police, K9 unit dogs all have an official police rank: PD (police dog). The handler gives evidence in court, something like "We gave chase, and PD Rover apprehended the suspect." PDs don't officially make an arrest because they cant read the suspect his/her rights!
