I’ve always hoped there was a reason behind it besides the fact that police officers are into chatspeak.
Wikipedia’s entry is sort of mixed:
Perhaps not a definitive source, but allwords.com’s entry confirms “k-9” = “canine.” Personally, I’d be surprised if the etymology were anything else.
People into having sex with dogs also use “K9” . . . according to a lot of the spam I keep getting.
And then there’s the Doctor’s companion, companion, companion.
(There were three, right? Well, four, but the last was a replacement after an explosion.)
Are you sad that police would use such a pun? Doesn’t the military use K9 too?
Dunno if it true but I heard K-9 is a dental reference. One of a dog’s main fangs is labelled K9 in dentistry and it fits with canine.
Interestingly I have recently begun to see K9 used in Spanish where it makes no sense unless you know it is taken from English and what it means.
That’s backwards. Canine teeth (humans have them too) are so called because they resemble a dog’s fang. If they’re labeled K9 on an X-ray it’s because it sounds like canine.
Huh?
Doctor Who.
Oh. Wiki sorted me out. (I’ve only seen the new ones)
What are the other teeth labeled?
Incisors, bicuspids, tricuspids, molars.
lmao.
I had to ask. I was expecting K3 or E9 or something. Note to self: Dentistry =/= Battleship.
Actually,
For some purposes: Dentistry = Battleship. 
CMC fnord!
The truth can never just be simple, eh?
Ok, but to clarify, I take it “K9” is still slang for “canine”, even in dental context, because the “dental formula” only goes up to 4, right?
For placental mammals the maxs are I3/3 C1/1 P4/4 M3/3, non-placentals (marsupials) I5/4 C1/1 P3/3 M4/4.
CMC fnord!