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1.
1. (verb) To perform an action prematurely, such as when Jimmy Fallon would laugh before the punchline, ruining many an SNL sketch.

2. (noun) Possibly the biggest, most inept cocksucker to ever come out of Saturday Night Live. Everyone who loved him has shunned him Amish Style. Commonly referred to as "The Man who somehow made a living with no actual talent", this fuckstick somehow convinced some dumb fuck of an executive to give him a fucking show. If you even so much as giggle at one of his attempts at humor, you have no soul.
Before the chick had a chance to pull it out, I Jimmy Falloned all over the inside of my pants.
by Fallon in Her Face May 02, 2009
 
2.
Jimmy Fallon is a former cast member of the popular NBC sketch comedy show Saturday Night Live. He is often referred to negatively as he often broke character or laughed during scenes. He went on to star in such films as Taxi alongside Queen Latifa, and Fever Pitch with Drew Barrymore.

Jimmy Fallon can also be used as a verb. When someone is expressing qualities that Jimmy Fallon often does, they would be "pulling a Jimmy Fallon."
Ben: "Hey Sean, did you watch Saturday Night Live last night?"
Sean: "Yes, it was pretty good, but I was a little distracted by Jimmy Fallon's hysterical laughing."
Ben: "Yeah, at first it was kind of funny when he forgot a line and started chuckling, but now it's getting a little old."

or

Ben: "And then the man went over to...*laughing*"
Sean: "Alright, cut. We have to keep starting the scene over because Ben's pulling a Jimmy Fallon."
Ben: "I'm sorry, I just find myself so entertaining."
by Ben Harris June 17, 2006
 
3.
Jimmy Fallon (Actor, Comedian, and so much more)

Jimmy Fallon was born September 19, 1974 in Brooklyn, NY. He started watching SNL at the age of 6, thats when his dream of one day being on the show started. Jimmy's first impression was of James Cagney, which he did at the age of two. He would go around saying, "you dirty rat." He ended his SNL job in 2004 during the season finale. He has a movie called 'Taxi' coming up in October where he plays a cop that doesn't know how to drive.
Some things Jimmy Fallon was in:

Taxi(2004)
Anything Else (2003)
Band of Brothers(2001)
The Scheme(2001)
Sex and the Matrix (2000)
Almost Famous (2000)
SNL Fanatic (2000)
Saturday Night Live (1998-2004)
by Maryanne July 10, 2004
 
4.
A guy who saved NBC Late Night's sorry ass twice in one decade. First time was in 2000 by resurrecting Weekend Update (with much help from Tina Fey), turning it into a fun and clever guy-girl exchange that worked wonders. Second time was in 2009 after the whole Leno-Conan reshuffling clusterfuck didn't work out, leaving Fallon as NBC's only non-fail comedian (out of four) still standing.

Writes jokes that are smart, relevant, and actually funny (unlike Leno). Also unlike Leno, his comedy sketches actually reflect some effort and creativity (i.e. Remix the Clips) instead of being tired ripoffs or just mooching off user input for humor. If there were justice in the world, Fallon would host the Tonight Show and Leno would get bumped to 4:03 AM on Telemundo.

A pretty good impressionist and parody songwriter, too.
Conan at his peak was still the best, but as replacements go, you gotta admit this Jimmy Fallon motherfucker is aiiiiiiite
by CrunchyCookie November 12, 2010
 
5.
the retard who doesn't deserve to be on Saturday night live, he laughed in every skit he was ever in literally. if you ever encounter him punch him as hard as you can.
by Crazy Cody 112 October 28, 2010
 
6.
Former cast member from SNL. Extremely talented with impersonations. He would break character often, but it would make the skit funnier, since SNL is pretty lame these days. He is the reason Tina Fey(even though she is hot) doesn't look like an idiot on Weekend update since she ruins her lines all the time. Hopefully he will be in a movie that will show off his talents like Ace Ventura did for Jim Carrey.
"And WE'RE BACK!!!!!!!!!!!!!!!!!!!!!!!!"
by Oklahoma smells like shit June 19, 2004