Having a nightmare but in the day, so it's just a mare since it's in the day.
Basically, another way of saying you're having a living nightmare, ie you're in a really shitty situation.
Today's been AWFUL. I thought my English Literature exam was in the afternoon, but it was actually in the morning. I missed the exam, and now I've been told I've been held back. It's like all my exam-anxiety nightmares came true! I'm truly having a mare!
Often used when things have gone completely tits up and you have no control over what the fuck is going on.
Small "mares" are usually caused by physics practicals, but larger ones are usually due to women.
The Ultimate Mare: Going out with a fit girl for two years, splitting up while living with her, sleeping with her friend, sleeping with ex again a few weeks later, pulling another of your ex's friends a few weeks after, then telling your ex the gory details including the fact that you did not play it safe with either and that she may now have got de baad AIDS.
Ruler of Lake Titicaca. Rumored to have a bunghole that gets very angry if it does not receive toilet paper. Cornholio the Great is often seen walking around with his shirt over his head and his hands in the air, chanting songs about his power, and his bunghole.
"I am Cornholio! You do not want to face the wrath of my bunghole, for I need TP!"
Butthead: Shut up, Beavis! (uh huh huh huh)
Beavis: Um, okay. (heh heh heh heh).