Green’s Dictionary of Slang

a horrendous situation, person or place.

2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 105: I had a bit of a mare with my kicking, missing seven out of my twelve kicks.
2005		P. Howard Curious Incident of the Dog in the Nightdress 57: What with Conall Gillen, our stand-in scrumhalf, having the mare that he did.