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Can occur on a Sunday evening bust most often Monday through Thursday. Caused by the excess consumption of alcholic beverages the previous evening, resulting in a craptastic hangover the following morning, most certainly resulting in a "sick" call into the office.
Dude, what happened to you yesterday, I didn't see you in the office, are you OK?

I got so shitfaced the other night that I had to call in sick this morning on account of the Beer Flu.
by C.A. Stallworth April 05, 2006
When you wake up the next morning after a heavy night of drinking, unable to go to work you phone and call in “sick”
John phoned in sick today, but I recon he has a case of beer flue.
by Linus Hanger May 26, 2006