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1.
A sickly and unidentifiable disease brought on by chronic marijuana use. Often augmented by nasty ass Whitman "beeshe(s)" and regularly going twenty deep.
"Hey man you wanna hit this "spleef?" Naw dude im on hiatus I have the plague. I just coughed up blood and urinated green today.
by BatmanB May 23, 2007
 
2.
in a historical context, the plague often refers to the bubonic plague or "black death", also known as pestilence, which wiped out large portions of the population of Europe and Asia during the 14th and 15th centuries

in a modern vernacular context, the plague refers to someone being out with illness, most often a chest cold or flu, usually exaggerating the extent of one's illness while demonstrating their incapacity
Sara can't come in today, she's out with the plague
Okay then send her my condolences and tell her to get better soon!

The Bubonic plague, otherwise simply called the "plague", killed millions of people in medieval Europe
by superblah March 30, 2013
 
3.
Contrary to common misconception this infact means very good or amazing. One is often amazed at the amazingness of something when using this phrase.
Guy 1: "I just won a million pounds.."
Guy 2: "zOMG, DAT IS THE PLAGUE!111! WANNA BE BFFL M8!!?"
Guy 1: "FU bish"
by Madame Bugatti August 25, 2008
 
4.
Posers. People who are not being themselves, and trying to fit in by acting like someone else. People doing things for popularity and so on.
Shit... here comes the plague.
by Corey Taylor March 27, 2005