You’re not supposed to get it! They never mention enough of the joke for the viewers to ever get it. That’s the beauty of it, it’s an orphaned punchline used to cut to the reactions of the characters and grab the viewers attention. There’s more about the writers technique here!
Now the interwebs claims the actual joke goes as such: So a penguin and a farmer walk into a bar. They sit next to a drunk guy and an unconscious bargoer. A drunk man asks the penguin what hes drinking. The penguin doesnt answer, he’s a penguin he can’t talk of coarse. The farmer watches finding the humor in the situation. Again the drunk man asks him, ” Whatchu drinkin?” The penguin still doesn’t reply. Confused the drunkin man asks, “What are you sir? A man? An alien? Ahh I know what you are.. You’re round and purple…You’re an eggplant huh? So whatchu drinking eggplant? With still no reply the drunkin man furiously screams, “WHAT ARE YOU DRINKING YOU ******* EGGPLANT?” The whole room stops and looks at the wild man and the penguin, when the unconscious drunk finally wakes up and says “Dude, he’s not an eggplant. He’s retarded!