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1.
1 : literally, the fifth of May

2a : a holiday celebrated by millions of Mexican nationals in America with notable festivity and drunkenness to commemorate their heritage and pride in the greatness of a country they couldn't wait to leave

2b : a holiday celebrated in Mexico by a small number of Mexicans with overwrought sadness and drunkenness to lament their expulsion from America by the Border Patrol <see also, el día de los muertos>
We wildly waved the la Bandera de México at the East LA Cinco de Mayo festival to protest those nasty gringos who won't defund the Border Patrol so they can afford to increase our welfare checks and medical care.

"Happy Cinco de Mayo! Now go kill some Gringo Bastardos!" Jose Angel Gutierrez
by J. Myars May 05, 2009
 
2.
A beer-fest that occurs on May 5 of the year, with lots of tequila and margaritas.
Man, I got so wasted on Cinco de Mayo!
by melissa morton May 10, 2003
 
3.
This started out as a tragedy in the late eighteenth century. It is a well known fact that mexicans love mayonnaise. A large steam tanker of this condiment was crossing the gulf of mexico from florida when a large storm occured, sinking the tanker. The horrified mexicans witnessed this and named the day of the sinking, May 5, a national holiday of drowning their worries with corona.
"O no-o! Did-o you hear-o about the cinco de mayo??!!"
by G Love July 06, 2006
 
4.
When a whole bunch of white college kids get together and have a kegger for reasons unbeknownst to them. These parties usually occur in the whitest of whitebread neighborhoods, and shirk far, far, farrrr away from what the people of Mexico actually celebrate.
Frat Boy A: Dude! You comin' to the Cinco De Mayo party tonight? We're gonna get WASTED!

Frat Boy B: Dude: I'm Mexican. That's effed up.

But I'm definitely coming.
by jimenarocker May 06, 2009
 
5.
If you don't know what cinco de mayo is, it's a celebration of Mexico's 1862 victory in battle over France, although France, who previously had not won a war since 1066, later won the war against Mexico. However, since the Mexicans beat the French for this one battle, Mexicans took their cue from the Irish (who don't actually celebrate St. Patrick's Day, from what I can tell). The Mexicans said, "Okay, hey, let's do a pub crawl!!!!!' Except they said it in Spanish. Which makes me wonder why Mexicans would do a pub crawl, seeing as how Mexico doesn't seem to have a lot of pubs, seeing as how it's Mexico and not Ireland. But never mind that now. You should also never mind the fact that cinco de mayo is more widely celebrated here in the States than it is in Mexico.
It's cinco de mayo! Let's get drunk for no reason!
by Tom Navarro April 18, 2007
 
6.
A time when all of LL posts mexican jokes on the boards and then faps.
Cinco De mayo consists of 1) a joke, then 2) fapping
1)joke(eg.):
Q: What were the 2 Mexican FireFighting Brother's names?
A: Hose A and Hose B

2)fapping, that is self explanatory
by llamaguy May 04, 2007
 
7.
A mayonnaise extract that is a rare delicacy in the southern United States. It can also be used as a sobriety test.
1) "Hey Jon, how's the burger?"
"It's good, but we should put some Cinco de Mayo on this bitch."

2) "Yo you wanna go to that Cinco de Mayo party?"

"What? What's THAT?"

"Oh, it's this new thing that they get out of mayonnaise. All the top chefs are using it in their gourmet cooking."

"No wayyyyyy! Yeah, let's go!"
by inspiredefined May 06, 2011