I just read this article in Buzzfeed: Garfield Creator Jim Davis Denies That Jon Drinks Dog Semen In A 1990 Comic Strip and apparently the online hubub started here on the Straight Dope.
Of course if you look at that thread and you look at Davis’s explanation at the end of the Buzzfeed article, **Miller **figured out the answer in one post.
But, we were cited today. Huzzah.
I’m feeling an absurd level of satisfaction over this.
Ooorrrr. . . Davis actually meant semen but then when it got this halfway disgusted reaction he saw Miller’s early explanation and ran with it.
And the rest of your answer, the “steamin cup of fido jizz” is immortal.
Dennis
Poculum seminis canum vaporans!
Ooh, is it talk like a pirate day already? Arrr! Ahoy there me mateys!
On topic. I think the comic makes a lot more sense if it’s dog semen.
Arrr, you’re about a month late for TLAPD.
According to Davis, we were supposed to clue in on the fact that Jon was having a healthy litter of puppies, not that Jon was having a healthy litter of puppies.
This might be a case of being too close to the subject matter to really see the joke. 
We will come up with the correct answer to anything, even if it takes eleven years!
:: wags tail ::
Is no one going to comment on the Twitter user who got the signed strip from Davis? No one? @realnutsling, whose bio reads “men’s microbikini enthusiast, power bottom and proud #Christian father of the three most beautiful girls in the world”? Ok.