Something said by obnoxiously cheerfull bastards when someone is feeling
tired and/or cranky. Mondays tend to be a horrible day since it's the first day of the working week.
Made famous by the movie Office Space.
Peter Gibbons: Let me ask you something. When you come in on
Monday, and you're not feelin' real well, does anyone ever say to you, "Sounds
like someone has a case of the Mondays"?
Lawrence: No. No, man.
Shit, no, man. I believe you'd get your ass kicked sayin' something like that, man.