Wikipedia has a list of neologisms on The Simpsons. Here are some of my favorites:
Car HoleHT Samantha Burns
A common man's term for garage, coined by Moe Szyslak in "The Springfield Connection". While "Car Hole" appears only twice in the series itself, it is often used by fans to jokingly refer to a garage, or garage-like structure.
The phrase first appears in a conversation between Moe Szyslak and Homer Simpson, wherein Moe ridicules Homer for his use of the overly formal word, "garage".
Homer: Hmm. I wonder why he's so eager to go to the garage?
Moe: The "garage"? Hey fellas, the "garage"! Well, ooh la di da, Mr. French Man.
Homer: Well what do you call it?
Moe: A car hole!
Another name for the cigarette lighter power socket. Used by the automotive appliance salesman when asking Homer what he had plugged into his Dash Hole.
A phrase used by Homer to describe food (positively) during his brief stint as a food critic in the episode "Guess Who's Coming to Criticize Dinner?":
Lisa: Hmm. What's the English equivalent for [drools like Homer]? I'd say...transcendent.
Homer: How about groin-grabbingly transcendent?
Lisa: Uh ... I don't think so.
Kwyjibo is a fictitious word made up by Bart Simpson during a game of Scrabble with his family. In the episode "Bart the Genius," Bart puts "Kwyjibo" on the board, scoring more than 150 points. When Homer demands Bart say what a Kwyjibo is, Bart replies, "A big, dumb, balding North American ape… with no chin." Marge adds in, "…and a short temper." At this point, Homer chases Bart away, causing him to exclaim, "Uh oh! Kwyjibo on the loose!"
Another name for a turkey. In "Lisa v. Malibu Stacy," Grandpa Simpson drones on with a rambling story about Thanksgiving:
"I just used [my washtub] that morning to wash my turkey, which in those days was known as a walking bird. We'd always have walking bird on Thanksgiving with all the trimmings: cranberries, injun eyes, yams stuffed with gunpowder. Then we'd all watch football, which in those days was called 'baseball'..."