A phrase used to describe someone that has a bad mood when he or she comes to work or school on Mondays.
OR
A phrase that someone uses if he or she wants to get their ass kicked by their fellow co-workers or school mates.
> Jerk #1 - When you come to work on a Monday, has anyone ever told you, "Looks like someone's got the case of the Mondays."?
> Jerk #2 - No... No man... Shit, no man... I believe you get your ass kicked for saying something like that.