It's a punchline to an old joke.
Guy of the homo persuasion was walking down an alley when he sees an homeless man passed out, pants slid halfway down his ass. Homo can't help himself, so has his way with passed out drunk. Afterwards, he feels bad, so he leaves a $20 in his pocket.
Homeless guy wakes up next morning, finds the $20 and promptly heads to the liquor store and buys a bottle of Mad Dog 20/20 (the cheapest stuff on the shelf), and promptly passes out again.
Shortly thereafter, Mr homo walks by again, finds drunk guy passed out again, and has his way, and leaves another $20 out of guilt. Homeless wakes up next day and gets his bottle of MD and passes out AGAIN!
Mr homo comes down the alley again. Repeats his actions, again. This time, being Christmas Eve, he leaves the guy a $50 out of guilt.
Homeless wakes up, finds the $50, heads to the liquor store before it closes. The clerk, familiar with him from the previous two days says: "let me guess, bottle of mad dog 20/20?".
Homeless guy says:"No. Give me a bottle of Jack. That cheap stuff makes my ass hurt..."