Green’s Dictionary of Slang

1. a dog.

[1857		Bell’s Life in Sydney 2 May 3/4: 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].
1986		C. Hiaasen Tourist Season (1987) 348: The police were trotting now, yellow-fanged K-9 dogs at their heels.
1998		L. Stringer Grand Central Winter (1999) 47: Police made rounds every night [...] armed with snarling, German shepherd K-9s.

2. a corrections officer.

2000

Other Side of the Wall: Prisoner’s Dict. July 🌐 K-9 Corrections officer (canine).