Green’s Dictionary of Slang

(Irish) of a young woman or girl, unattractive.

2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 64: Ms Cully – we’re talking Bet-Down City, Idaho here.
2005		P. Howard Curious Incident of the Dog in the Nightdress 2: Anto had this sister, Tina, who was, like, bet-down. I mean, the focking tide wouldn’t take her out.