Breaking character. Laughing or going out of character while performing an act. Name comes form comedian Jimmy Fallon who is famous for laughing in most of his sketches from Saturday Night Live and his late night shows.
Brody: "Man, Kyle was awful last night"
James: "Dude kept Jimmy Falloning"
by Marilyn Hardy April 26, 2014
When you laugh at every joke.
"I can laugh at anything"
"You're basically a Jimmy Fallon then"
by 3du April 19, 2019
A Jimmy Fallon is where you laugh, clap, lie back onto your chair and smack the table a dozen times then immediately stop laughing.
Conan O'Brien: And then my parents hated me-
Stephen Colbert: 不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不 *banging on table* 不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不 *clapping* 不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不不 no.
Conan O'Brien: *Whispers to Jimmy Kimmel* He is such a Jimmy Fallon.
by Adrianne0620 December 20, 2019
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 18, 2006
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 3, 2009
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
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