not a clue what k9 means...help?

Doggy Style.

K9 is having sex with a dog or allowing a dog to go down on you. I think that is so wrong .

And you aren't either.  But it's more common to see someone say "we did k9" (doggystyle) than it is to see someone say "I did a k9" (dog).

It's in front of the molars.