Phrase. Short for "Having a nightmare".
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.